fix(cli): close 7 review findings on default subcommand + --command

[brettdavies]

Summary

Resolves the seven recommended fixes from the post-merge /ce-review of PR #12 (commit
4afef67). All actionable findings closed; main.rs trimmed back under the 200-line trigger.

Changelog

Added

• value_hint = ValueHint::CommandName on --command so zsh, fish, and elvish completions
  suggest PATH commands instead of file paths. Bash is patched post-generation in
  scripts/generate-completions.sh.
• after_help text on Cli documenting the implicit default subcommand and the
  bare-invocation contract directly in anc --help output.
• Mutual exclusion: --command and --source now error at parse time instead of
  silently producing an empty result.

Changed

• anc -q / anc --quiet (top-level flag without subcommand) now prints help and exits 2
  instead of panicking via unreachable!() (pre-existing bug).
• anc help and anc help check now work — clap's auto-generated help subcommand was
  missing from our known-subcommand set and got misclassified as a path.
• anc --command <NAME> where NAME collides with a subcommand name (e.g.
  anc --command check) now resolves NAME as a binary on PATH instead of producing a
  confusing clap error.
• anc --command rg and anc --output json --source (no positional argument) now work —
  the pre-parser detects subcommand-scoped flags and injects check accordingly.
• anc -- . (POSIX double-dash separator) now runs check against . instead of
  producing undefined behavior.

Documentation

• README and AGENTS.md exit-code tables clarify that exit 2 is overloaded (failures,
  errors, and usage errors all share it). Suggest parsing stderr (Usage: text) to
  distinguish.

Type of Change

• fix: Bug fix (non-breaking change which fixes an issue)

Related Issues/Stories

• Original feature: PR feat(cli): default check subcommand and --command flag for PATH lookup #12 (Plan 003 merge, commit 4afef67)
• Review run artifact: .context/compound-engineering/ce-review/20260415-173344-efaaf5ff/
• Plan: docs/plans/2026-04-02-003-feat-cli-default-subcommand-and-command-flag-plan.md

Findings Resolved

#	Severity	Title	Resolution
1	P2	clap help subcommand missing from known set	Append \"help\" to known after get_subcommands()
2	P2	--command <NAME> value collides with subcommand	Pair value-taking flags with their values via clap introspection
3	P2	Flag-value tokens silently fragile	Same fix as #2; also tracks subcommand-scoped flags to inject check when no positional follows
4	P2	anc -q panics via unreachable!() (pre-existing)	None arm now renders help to stderr and exits 2
5	P2	--command + --source silent empty result	conflicts_with = \"source\" on command arg
6	P2	Bare anc exit 2 conflates user-error vs check-failure	Documented in README/AGENTS exit-code tables
7	P2	Default-subcommand magic not in --help	after_help block on Cli documents it
8	P2	anc -- . POSIX double-dash undefined	New branch in inject_default_subcommand
10	P2	main.rs over 200-line refactor trigger	Extract inject_default_subcommand + tests to new src/argv.rs module (538 → 203 lines)
14	P3	Bash --command completion uses compgen -f	ValueHint::CommandName fixes zsh/fish/elvish; bash post-patched in gen script
22	P3	anc . succeeds where it previously errored	Documented in PR Changelog above

Findings not addressed (intentional): #11/#12/#17/#18 (subsumed by #10 refactor), #15/#16/#19/#20/#21/#23/#24
(low-priority test/style nits or pre-public-schema concerns).

Testing

• Unit tests added/updated (244 → 253, +9 covering all 5 fixed edge cases)
• Integration tests added/updated (26 → 34, +8 covering same)
• All tests passing
• Pre-push hook clean (fmt + clippy -Dwarnings + test + Windows compat)
• Manual probes: every previously-broken invocation now does the right thing,
  bare anc still exits 2 (fork-bomb guard intact), explicit subcommands still work

Test Summary:

• Unit tests: 253 passing (was 244)
• Integration tests: 34 passing, 3 ignored (was 26 / 3)
• Coverage: every fix has both a unit test (where applicable) and an integration test

Files Modified

Modified:

• src/cli.rs — after_help text, value_hint on --command, conflicts_with = \"source\"
• src/main.rs — None arm prints help instead of panicking; refactor extracts injection
  to argv module
• tests/integration.rs — 8 new tests covering the fixed edge cases
• scripts/generate-completions.sh — post-process bash to substitute compgen -c for
  --command (clap_complete bash backend ignores ValueHint::CommandName)
• completions/anc.{bash,zsh,fish} — regenerated to expose new behavior
• README.md, AGENTS.md — exit-code clarification

Created:

• src/argv.rs — new module hosting inject_default_subcommand and 19 unit tests

Breaking Changes

• No breaking changes

Every previous valid invocation continues to produce identical output. Previously-broken
invocations (anc help, anc -q, anc --command check, anc --command rg,
anc --output json --source, anc -- .) now behave correctly.

Deployment Notes

• No special deployment steps required

Ships with Plan 003 in the v0.1.0 release/* batch when Plan 002 (release infra) runs.
No standalone release branch.

Checklist

• Code follows project conventions and style guidelines
• Commit messages follow Conventional Commits
• Self-review of code completed
• Tests added/updated and passing
• No new warnings or errors introduced
• Changes are backward compatible