fix(cmd): correctness + audit hardening across cmd/ + internal/

Addresses audit findings from a tri-partite review (codex + 2 Claude agents).
Multiple distinct fixes here because they touched overlapping files; happy
to split via interactive rebase if a reviewer prefers.

## Correctness bugs (HIGH)

* `--config` is now actually honored. cmd/root.initConfig fed Viper but
  every command that mattered loaded config via `internal/config.Load()`
  which always read the default path. Added `config.SetExplicitConfigPath`
  consulted by `GetConfigPath`; `--config other.yaml auth login` now writes
  to other.yaml.
  - internal/config/config.go, cmd/root.go

* `--json` now works on `fj run …`, `fj workflow …`, and `fj wiki view`.
  cmd/aliases.go registered `--json` as a Bool but the handlers call
  `wantJSON()` which does `GetString("json")` and silently ignores the
  type-error return. cmd/wiki.go did the inverse (`GetBool("json")` against
  a string-registered flag). Both now use `addJSONFlags`/`wantJSON`/
  `outputJSON` consistently.
  - cmd/aliases.go, cmd/wiki.go

* `fj api` no longer lets endpoints escape the /api/v1 base via
  path-traversal. `fj api '/../admin/users'` previously normalized to
  `/admin/users` because `http.NewRequest` resolves `..` segments —
  silently sending authenticated traffic to non-API routes. Endpoint is
  now parsed, `..` segments are rejected, and JoinPath is used.
  - cmd/api.go

## Design rework (BREAKING — gets rid of the `--json=fields` quirk)

* `--json` flag rebuilt from a string-with-NoOptDefVal=" " sentinel into a
  plain Bool. `--json-fields` keeps comma-separated projection. The two
  are mutually exclusive (`MarkFlagsMutuallyExclusive`). `--jq` composes
  with either or neither. The previous design produced a `--json string[=" "]`
  in --help and required `--json=fields` (with literal "=") because
  `--json fields` was parsed as the bare flag plus a positional. Gone.
  - cmd/json.go: addJSONFlags / wantJSON / outputJSON
  - cmd/api.go: example block reflects the new shape

  Migration: `--json=fields` → `--json-fields fields`. Bare `--json` still
  means "everything as JSON".

* `fj api` now uses `internal/api.SharedHTTPClient` (30 s timeout, pooled)
  instead of constructing a zero-value `&http.Client{}` with no timeout.
  A hung Forgejo no longer pins the CLI indefinitely. Response body is
  also bounded by `io.LimitReader` at 64 MB to prevent OOM-on-self.
  - internal/api/client.go (export SharedHTTPClient), cmd/api.go

* `--hostname` declared as a persistent flag on rootCmd is now the only
  declaration. cmd/auth.go re-declared `--hostname` on three subcommands,
  shadowing the persistent flag — meaning `fj --hostname=X auth login`
  and `fj auth login --hostname=X` went through different code paths
  (viper read vs. local flag read). Local declarations removed.
  - cmd/auth.go

## Hardening (MEDIUM/LOW)

* `--token` on `auth login` now emits a stderr warning when used, since
  it puts the PAT on argv (visible in `ps auxe`/shell history). Flag not
  removed — too disruptive — but discoverable now.
  - cmd/auth.go

* Error handling no longer regex-matches "401"/"403" against rendered
  error strings (would have triggered "auth login" hint for an error
  that just mentioned issue #403). Now relies on typed `*api.APIError`.
  Hints moved to a separate `Hint` field on `CLIError`, so JSON-error
  consumers get clean structure and the human renderer still appends
  "\nHint: …".
  - cmd/errors.go

* `migrateConfigDir` now opens dst with `O_TRUNC` instead of just
  `O_CREATE|O_WRONLY`. Previously a partially-pre-existing dst file
  would have legacy contents overwrite a prefix and leave stale tail
  bytes — silent YAML/token corruption.
  - cmd/root.go (extracted into copyOneConfigFile with proper close handling)

* Config dir created with mode 0700 instead of 0755. `initConfig` warns
  on stderr if the resolved config file is world/group readable
  (`mode & 0o077 != 0`); doesn't fail-close.
  - cmd/root.go

* Network errors (`no such host`, `connection refused`, `i/o timeout`)
  now return a structured `CLIError` with code `ErrNetworkError` and a
  hint, instead of a fmt.Errorf chain.
  - cmd/errors.go

Verified: `go build ./...` and `go test ./...` clean. Live integration
tested against forgejo.zerova.net.

Out of scope, deferred to follow-up commits:
- Pagination unification across `repo list`/`pr list`/`issue list` (only
  `release list` walks pages today; others silently truncate).
- `fj api --paginate` to follow pages like `gh api --paginate`.
- De-duplicating cmd/aliases.go ↔ cmd/actions.go subtrees.

cmd/root.go

@@ -8,6 +8,7 @@ import (
 	"strconv"
 	"strings"
 
+	"forgejo.zerova.net/public/fj/internal/config"
 	"forgejo.zerova.net/public/fj/internal/git"
 	"github.com/spf13/cobra"
 	"github.com/spf13/viper"
@@ -45,7 +46,12 @@ func init() {
 
 func initConfig() {
 	if cfgFile != "" {
+		// Tell viper to load this file for env-style overrides AND make
+		// internal/config.Load()/.Save() use it (this is the load-bearing
+		// half — without SetExplicitConfigPath, --config was silently
+		// ignored by every auth-touching command).
 		viper.SetConfigFile(cfgFile)
+		config.SetExplicitConfigPath(cfgFile)
 	} else {
 		home, err := os.UserHomeDir()
 		if err != nil {
@@ -66,7 +72,7 @@ func initConfig() {
 			}
 		}
 
-		_ = os.MkdirAll(configDir, 0755)
+		_ = os.MkdirAll(configDir, 0700)
 
 		viper.AddConfigPath(configDir)
 		viper.SetConfigType("yaml")
@@ -77,6 +83,14 @@ func initConfig() {
 	viper.SetEnvPrefix("FJ")
 
 	_ = viper.ReadInConfig()
+
+	// If the resolved config exists with overly permissive mode, warn — the
+	// file holds API tokens. Don't fail-close; just nudge the user.
+	if path, err := config.GetConfigPath(); err == nil {
+		if info, statErr := os.Stat(path); statErr == nil && info.Mode()&0o077 != 0 {
+			fmt.Fprintf(ios.ErrOut, "warning: %s mode %o is world/group readable; tokens may leak. chmod 600 it.\n", path, info.Mode().Perm())
+		}
+	}
 }
 
 // parseRepo parses the repository string in the format "owner/name".
@@ -143,8 +157,11 @@ func parseIssueArg(arg string) (int64, error) {
 }
 
 // migrateConfigDir copies all files from src to dst (one level, no subdirs).
+// Uses O_TRUNC so a partially-pre-existing dst file is fully replaced rather
+// than having the legacy contents overwrite a prefix and leaving stale tail
+// bytes — which for a YAML token store would silently corrupt config.
 func migrateConfigDir(src, dst string) error {
-	if err := os.MkdirAll(dst, 0755); err != nil {
+	if err := os.MkdirAll(dst, 0700); err != nil {
 		return err
 	}
 	entries, err := os.ReadDir(src)
@@ -155,21 +172,34 @@ func migrateConfigDir(src, dst string) error {
 		if e.IsDir() {
 			continue
 		}
-		in, err := os.Open(filepath.Join(src, e.Name()))
-		if err != nil {
-			return err
-		}
-		out, err := os.OpenFile(filepath.Join(dst, e.Name()), os.O_CREATE|os.O_WRONLY, 0600)
-		if err != nil {
-			in.Close()
-			return err
-		}
-		_, err = io.Copy(out, in)
-		in.Close()
-		out.Close()
-		if err != nil {
+		if err := copyOneConfigFile(filepath.Join(src, e.Name()), filepath.Join(dst, e.Name())); err != nil {
 			return err
 		}
 	}
 	return nil
 }
+
+func copyOneConfigFile(srcPath, dstPath string) (retErr error) {
+	in, err := os.Open(srcPath)
+	if err != nil {
+		return err
+	}
+	defer func() {
+		if cerr := in.Close(); retErr == nil {
+			retErr = cerr
+		}
+	}()
+
+	out, err := os.OpenFile(dstPath, os.O_CREATE|os.O_WRONLY|os.O_TRUNC, 0600)
+	if err != nil {
+		return err
+	}
+	defer func() {
+		if cerr := out.Close(); retErr == nil {
+			retErr = cerr
+		}
+	}()
+
+	_, err = io.Copy(out, in)
+	return err
+}