feat(cli): add read-only agent-init status subcommand

[Lillevang]

Summary

• Adds agent-init status [target] — read-only reporter that prints the scaffold's current visibility mode (shared, local, hidden, or shadowed-by-global), the absolute path of the file carrying the agent-init ignore block, and the exact sed undo command. For shadowed-by-global it also prints the force-add hint explaining why files may still be tracked.
• Detection follows git's ignore precedence (.gitignore > .git/info/exclude > core.excludesfile) and reports the most-local carrier. Writes nothing; touches no git config.
• Reuses the marker constants and path helpers from internal/gitignore rather than duplicating them. gitignore gains HasBlock and exports MarkerStart/MarkerEnd so the detector and the undo hint stay byte-for-byte in sync with the writers.
• Wired into the help table in internal/cli/cli.go; TestHelpFlagsMatchDocs, the top-level help test, and the flagless-subcommand help test are updated to include status. Docs added under docs/cli.md; README subcommand list updated.

Closes #60.

Acceptance criterion calls

• Mode names: used shared, local, hidden, shadowed-by-global verbatim from the issue.
• Precedence when block exists in multiple files: report the most-local hit (.gitignore > .git/info/exclude > global). shadowed-by-global only fires when neither repo-local file carries the block but the global one does — the state where the scaffold is committed openly here yet ignored everywhere else.
• Undo command: portable sed -i.bak invocation that works on both GNU and BSD sed, plus a note about the .bak leftover and a manual-edit fallback. Markers come from gitignore.MarkerStart / MarkerEnd.
• Detection is marker-based, not effective-behavior: documented explicitly under docs/cli.md "Behavior". A user who manually negates a rule below the block still sees local/hidden/shadowed-by-global.
• No flags: rejects unknown flags (status --no-such-flag errors instead of silently treating the flag as a target path).

Test plan

• ./.agent/scripts/check.sh passes (codemap, fmt, lint, lint-shell, mod-tidy, typecheck, test, cross-build, smoke-test).
• just smoke-test passes (no template changes, but verified — no-op).
• New unit tests in internal/cli/status_test.go cover: shared, local, hidden, shadowed-by-global, multi-carrier precedence, default target (cwd), explicit target canonicalization, extra-positional rejection, unknown-flag rejection, three help triggers, and graceful degradation when HOME is unreadable. Read-only is asserted via on-disk snapshots taken before and after the run.
• New gitignore tests cover HasBlock (six edge cases) and a drift guard linking MarkerStart/MarkerEnd to the internal constants.
• Manual smoke: scaffolded a local and a hidden project and ran status against both; output matches the documented format.

🤖 Generated with Claude Code

[Lillevang]

LOW — internal/cli/status.go:165 (writeStatusReport): the carrier path is printed unquoted into the suggested sed -i.bak '/.../,/.../d' <path> undo command. The path is derived from the positional target arg (or, in shadowed mode, from git config --global core.excludesfile). A target like '; echo pwned; # would render an undo line that, if pasted into a shell, runs the injected command. Blast radius is bounded — the user controls both the arg and the paste — but the line is presented as a copy-paste recipe, which earns it a hardening note.

Fix: render the path with %q (or strconv.Quote) so the printed command is sed -i.bak '/.../,/.../d' "<path>" and metacharacters are inert when pasted. Apply the same to the two trailing "or open ..." lines below.