fix(source-quality): eliminate unwraps + route stdout through src/output.rs

[brettdavies]

Summary

Dogfooding anc check . against this repo on the v0.2.0 binary surfaced one FAIL and one multi-site WARN that
together pulled the project-mode score down to 90%. This PR addresses both, taking the score to 97% with 0 fail
/ 1 warn (the remaining warn is p2-json-output's known probe limitation: --help/--version short-circuit
--output flags, so safe probes can't validate the JSON envelope; that's a check-design concern, not a real
regression).

Two related cleanups, plus the supporting refactor:

1. Unwrap fail (4 sites → 0). Three .unwrap() sites are infallible by upstream contract: clap's
   ValueEnum::to_possible_value() always returns Some for declared variants (src/skill_install.rs:574, 613), and
   Path::file_name() / OsStr::to_str() on a glob-matched ASCII filename in build.rs cannot fail in practice. All
   three are now .expect("…") calls naming the invariant in source so the next reader doesn't have to reconstruct
   the reasoning.
2. Naked println warn (11 sites → 0). Two distinct cases bundled here:
   • build.rs (9 sites). Cargo build scripts emit metadata via println!("cargo:rerun-if-changed=…") and
     println!("cargo:warning=…") directives; there's no other API. The p7-naked-println check now exempts
     build.rs at any crate root via a precise path-segment predicate (rejects misnamed src/build.rs,
     tests/build.rs, etc).
   • main.rs:259 + skill_install.rs:510 (2 sites). Both are legitimate stdout hand-offs at the orchestration
     boundary. New src/output.rs centralizes them behind output::emit (no trailing newline) and
     output::emit_line (with newline). The check's existing output|display in path exemption already covers the
     new module by convention, so no check change is needed for this case.

Changelog

Changed

• p7-naked-println source check now exempts build.rs at any crate root. Cargo build scripts use
  println!("cargo:…") directives by protocol; flagging them produces noise without an alternative API. Misnamed
  src/build.rs or tests/build.rs files stay flagged.

Fixed

• Eliminated four .unwrap() calls on infallible operations across src/skill_install.rs and build.rs. Replaced
  with .expect("…") naming the upstream contract that guarantees Some/Ok. No behavior change; these were
  already infallible, and the expect messages document why.

Type of Change

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

Related Issues/Stories

• Story: dogfooding the v0.2.0 binary against this repo to catch source-quality regressions before they reach Show HN readers.
• Issue: n/a (surfaced via anc check . dogfood, not a filed issue).
• Architecture: src/output.rs becomes the single stdout hand-off point for the binary; src/checks/source/rust/naked_println.rs exempts build scripts at any crate root.
• Related PRs: sibling PR chore(release): add sync-dev-after-release.sh + document post-publish backport #37 (chore/sync-dev-after-release) ships the release backport script + RELEASES.md step. Both branches ride today's release together.

Testing

• Unit tests added/updated
• All tests passing

Test Summary:

• Unit tests: 535 passing (2 new tests in naked_println.rs covering is_cargo_build_script predicate: positive cases bare/relative/absolute/Windows/case-insensitive; negative cases src/, tests/, examples/, benches/ subtrees and build.rs.bak; plus an integration check that the helper still warns at unit level while run() filters by path).
• Integration tests: dogfood verification via anc check . reports 33 checks: 28 pass, 1 warn, 0 fail, 4 skip → 97% (was 90%). Binary mode unchanged at 89% (orthogonal to source-side findings).
• Coverage: cargo fmt --check, cargo clippy --release -- -Dwarnings, full pre-push hook all green.

Files Modified

Modified:

• build.rs: .unwrap().to_str().unwrap() chain → .expect("…") calls naming the invariant.
• src/checks/source/rust/naked_println.rs: is_cargo_build_script helper exempts crate-root build scripts; doc-comment on the module updated; 2 new tests.
• src/main.rs: declares the new output module; routes the final stdout hand-off through output::emit.
• src/skill_install.rs: two test-side .unwrap() → .expect() replacements; println!("{rendered}") → crate::output::emit_line(&rendered).

Created:

• src/output.rs: single stdout hand-off point with emit and emit_line helpers.

Renamed:

• None.

Deleted:

• None.

Breaking Changes

• No breaking changes

The naked-println exemption broadens what passes; no tool that previously passed will now fail.

Deployment Notes

• No special deployment steps required

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