feat(api): add --json, --json-fields, --jq to fj api

`fj api` was the only command that returned raw API JSON without exposing
the same projection/filtering knobs that `fj repo list`, `fj pr list`,
etc. already provide. Callers had to pipe to `python -m json.tool` or
`jq` to extract fields, which is inconsistent and discoverable only
after hitting the gap.

Wire the existing addJSONFlags / wantJSON / outputJSON helpers from
cmd/json.go so the API command participates in the same JSON output
pipeline. No behavioral change when none of the new flags are set —
default still pretty-prints JSON and writes raw bytes for non-JSON
responses.

Verified against live forgejo:

  $ fj api repos/public/claude-code-proxy --jq .full_name
  public/claude-code-proxy

  $ fj api repos/public/claude-code-proxy --json=full_name,description
  { "description": "...", "full_name": "public/claude-code-proxy" }

  $ fj api 'repos/public/claude-code-proxy/commits?limit=3' \
        --jq '.[] | "\(.sha[0:8]) \(.commit.message | split("\n")[0])"'
  8e550b97 Local fork: hardening + ops improvements ...
  b9da198e Harden proxy auth, storage, and conversation access
  6cda3631 Harden streaming, pagination, and config loading

Note: `--json=fields` requires the equals sign because the flag has
NoOptDefVal=" " (so `--json` alone is valid for "everything as JSON").
The Example block in --help documents both the `--json=` form and the
`--json-fields` alias which doesn't have that quirk.

cmd/api.go

@@ -35,7 +35,17 @@ If --field is used and no --method is specified, the method defaults to POST.`,
   fj api /users/johndoe
 
   # Use raw body from stdin
-  echo '{"title":"test"}' | fj api /repos/{owner}/{repo}/issues --input -`,
+  echo '{"title":"test"}' | fj api /repos/{owner}/{repo}/issues --input -
+
+  # Filter the response with a jq expression
+  fj api /repos/{owner}/{repo}/issues --jq '.[].title'
+
+  # Project the response down to specific fields (requires "=" because --json
+  # also accepts an empty value to mean "all fields as JSON")
+  fj api /repos/{owner}/{repo} --json=full_name,description,private
+
+  # Same projection without the "=" quirk
+  fj api /repos/{owner}/{repo} --json-fields full_name,description,private`,
 	Args: cobra.ExactArgs(1),
 	RunE: runAPI,
 }
@@ -50,6 +60,7 @@ func init() {
 	apiCmd.Flags().StringArrayP("header", "H", nil, "Add an HTTP request header (key:value)")
 	apiCmd.Flags().Bool("silent", false, "Do not print the response body")
 	apiCmd.Flags().BoolP("include", "i", false, "Include HTTP response headers in the output")
+	addJSONFlags(apiCmd, "Output the response as JSON; pass a comma-separated field list to project specific keys")
 }
 
 func runAPI(cmd *cobra.Command, args []string) error {
@@ -212,18 +223,31 @@ func runAPI(cmd *cobra.Command, args []string) error {
 		return nil
 	}
 
-	// Pretty-print JSON, or output raw if not JSON
 	contentType := resp.Header.Get("Content-Type")
-	if strings.Contains(contentType, "json") || json.Valid(respBody) {
+	isJSON := strings.Contains(contentType, "json") || json.Valid(respBody)
+
+	// If the user asked for JSON projection or jq filtering, route through
+	// the shared JSON output helpers so the API command is consistent with
+	// `fj repo list`, `fj pr list`, etc.
+	if wantJSON(cmd) {
+		if !isJSON {
+			return fmt.Errorf("--json/--json-fields/--jq requires a JSON response, but the server returned %s", contentType)
+		}
+		var parsed any
+		if err := json.Unmarshal(respBody, &parsed); err != nil {
+			return fmt.Errorf("response is not valid JSON: %w", err)
+		}
+		return outputJSON(cmd, parsed)
+	}
+
+	// Pretty-print JSON by default, otherwise emit raw bytes.
+	if isJSON {
 		var parsed any
 		if err := json.Unmarshal(respBody, &parsed); err == nil {
-			enc := json.NewEncoder(ios.Out)
-			enc.SetIndent("", "  ")
-			return enc.Encode(parsed)
+			return writeJSON(parsed)
 		}
 	}
 
-	// Raw output for non-JSON responses
 	_, err = ios.Out.Write(respBody)
 	return err
 }