feat: repo archive/unarchive + completion install

- fgj repo {archive,unarchive}: toggle a repo's archived state via
  EditRepo with *bool Archived. Archive prompts for confirmation
  (requires --yes in non-TTY envs); unarchive is reversible, no prompt.
  Accepts positional owner/name or -R flag; -R wins when both given.

- fgj completion install [shell]: idempotent install of completion
  scripts to shell-standard locations. Auto-detects shell from $SHELL
  if omitted. Paths: bash → XDG (or brew prefix on macOS), zsh →
  ~/.zsh/completions/_fgj, fish → ~/.config/fish/completions/fgj.fish.
  --system (bash only) prints the sudo command for /etc paths without
  writing. --dry-run prints the target path without writing.
  Compares existing-file contents before overwrite to stay idempotent.

Both files built by sub-agents in parallel. Build + vet + test clean.

CHANGELOG.md

@@ -82,6 +82,15 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
   deltas (`7d`, `24h`, `2w`, `1m`). Server-side filter for issues,
   client-side for PRs (SDK lacks a PR-side filter).
 - `fgj label update` added as an alias for `fgj label edit`.
+- `fgj repo {archive,unarchive}` — toggle a repository's archived state
+  via `EditRepo`. Archiving prompts for confirmation (requires `--yes`
+  in non-TTY environments); unarchiving is reversible and skips the
+  prompt.
+- `fgj completion install [shell]` — idempotently writes the
+  completion script to the shell-standard location (XDG for bash,
+  `~/.zsh/completions/_fgj` for zsh, `~/.config/fish/completions/fgj.fish`
+  for fish; brew prefix on macOS when present). Supports `--dry-run`
+  and `--system` (bash only, prints the required sudo command).
 
 ### Changed