Migrate commit validator to Rust

[sebastien-boulle]

Summary

Migrates the commit-message validator from bash to a single Rust binary (commit-message-validator), as designed in docs/superpowers/specs/2026-07-03-rust-migration-design.md.

Behavior-compatible port: identical validation rules and exit codes (Structure=1 … Revert=10), same env var names (COMMIT_VALIDATOR_*, GLOBAL_*) with empty/unset → off/default, same defaults (header 100, body 100, jira types feat fix). The .bats suites were ported to Rust as the golden reference.

Architecture

• Pure library core (no I/O): config, error, patterns, parser (state machine), validate (rules), preprocess.
• Thin clap CLI (main.rs) with two subcommands:
  • commit-message-validator message <FILE> — commit-msg hook (replaces check_message.sh)
  • commit-message-validator range <REV_RANGE> — CI / pre-push (replaces check.sh)
• Git accessed via subprocess (keeps the binary tiny and musl-static-friendly).
• Flags unified with env fallback (flag wins, then env, then default).

Distribution

• CI (.github/workflows/ci.yml): fmt + clippy -D warnings + test + cargo llvm-cov (hard-fails < 90%; currently 97.82%).
• Release (.github/workflows/release.yml): on v* tags, builds static binaries for {x86_64,aarch64}-{unknown-linux-musl,apple-darwin} and attaches them to the release.
• Composite Action (action.yml): downloads the matching prebuilt binary and runs range.
• pre-commit / pre-push: hooks/commit-message-validator shim downloads/caches the binary.
• Old bash scripts and .bats tests removed; README updated.

Testing

74 tests (unit + assert_cmd CLI integration against temp git repos). Built strictly TDD; each commit is atomic and independently green.

Review process

Built via subagent-driven development: a fresh implementer + spec/quality reviewer per task, plus a final whole-branch review. The final review confirmed behavioral equivalence with the bash original (and noted it even fixes two latent bash bugs: cross-commit global state leakage in ranges, and NO_REVERT_SHA1 aborting the whole range). Two cross-cutting bugs it found were fixed in b2e3616 (preprocess trailing-newline rejection of normal commit-msg files; download shim defaulting to a nonexistent master release).

Known follow-ups (draft — not blocking review, flagging for decision)

• Cut a v2.x release with the Rust binary assets before switching consumers over — the pre-commit/pre-push/Action integrations need published assets to work end-to-end (CI validates the code meanwhile). The shim now defaults to releases/latest.
• Checksum/signature verification of the downloaded binary (Action + shims) is not yet implemented — currently relies on TLS. Suggest publishing + verifying SHA256SUMS.
• Behavior note: the message path now honors ambient env vars (bash check_message.sh explicitly unset them). Intentional per the unified flag/env design; called out in case local-shell env should be ignored for local commits.
• Minor: README still has older per-integration option lists that partially duplicate the new unified options table.

See docs/superpowers/plans/2026-07-03-rust-migration.md for the full implementation plan.