feat(complexity-gate): harness package for the complexity gate

[ElbertePlinio]

Closes #87

What changed

• New @pickforge/complexity-gate package mirroring review-tutor: bin/complexity-gate wrapper (COMPLEXITY_GATE_BIN > vendored > PATH), checksum-verified postinstall downloader for the cargo-dist release archives, complexity-gate-install (--harness claude,codex,pi, --all, --print, interactive), Pi extension (edit feedback + turn-end gate with a 3-block loop guard), Claude plugin (PostToolUse/Stop hooks + skill) and marketplace entry, Codex hooks fragment + skill.
• Binary contract: github.com/pickforge/complexity-gate (docs/spec.md).

Tested

• bun run typecheck, bun run lint, bun run test (package tests incl. a real tar.xz extraction with the cargo-dist directory layout and *-prefixed checksum lines), bun run --cwd packages/complexity-gate build.
• Hook fragments exercised against the real binary in live Claude Code and Codex sessions (see v0.1: tree-sitter complexity gate binary with Claude/Codex hooks complexity-gate#1).

Not tested

• Windows download/extraction and pi.cmd execution (implemented, mocked only).
• pi install end-to-end (the extension type-checks against @earendil-works/pi-coding-agent; live Pi run happens after merge when the maintainer wires the package).
• A published release does not exist yet, so postinstall currently warns and skips; COMPLEXITY_GATE_BIN or a PATH binary is used until v0.1.0 is tagged.

Known risks

• Downloader assumes cargo-dist's sha256.sum and complexity-gate-<target>/ layout (verified locally with dist build for linux-x86_64).

Not release-note-worthy for app repos; no docs/releases/UNRELEASED.md in this repo.

Local-review risk class: standard (one correctness reviewer; P1 Windows-asset finding dismissed — cargo-dist emits .zip for Windows, confirmed via dist plan; two P2s fixed).