feat(skills): add memory-tidy, the judgment half of the memory plane

[tieubao]

What

Adds skills/memory-tidy: the judgment half of the kit's memory plane. The mechanical half already lives here (lib/stats memory-sweep, SPEC-136: read-only dead-ref + index scanner). This skill consumes that scanner's output and is the only path that edits a .claude/memory/ store: evidence-required per-note verdicts (KEEP / MERGE / STALE / UNSURE) via agent fan-out, danger check for notes whose advice contradicts current policy, index rebuild with parity checks, all confined to a worktree branch and shipped as a PR (the operator's approval gate).

Why

The scanner (kit) and the judgment (previously a personal dotfiles skill) split one concern across two repos, against the one-engine-one-truth rule. Generated-data curation (memory notes feeding promotion/backlog flows) belongs with the machinery that already scans it.

Test

Written per writing-skills RED/GREEN: against a fixture store with 4 planted defects (dead-ref note, duplicate pair, unindexed note, ghost index entry), the no-skill baseline deleted notes directly on the current branch and kept the dead-ref note unexamined; with the skill, the agent branched in a worktree, caught the dead ref with git log --all evidence, merged the duplicate pair quoting the overlap, rebuilt the index to parity, and stopped at the PR gate. First production use: tieubao/ops-toolkit#972 (118-note audit, manual run of the same process).

Consumer side (scheduler, biweekly launchd, vps-mon heartbeat) stays in ops-toolkit tools/memory-tidy/ (tieubao/ops-toolkit#975). After this merges: refresh the plugin snapshot and remove the interim dotfiles copy of the skill.

Second commit registers the component per the ask: [features].memory_tidy [consumer] toggle in kit.toml (single control surface; cadence stays consumer-side, default off), README Skill-roster row + tree line de-rostered to stop churn, docs/CHANGELOG.md [Unreleased] entry, and a new CONTRIBUTING "Introducing a component" section (naming, description discipline, registration, RED/GREEN evidence) so future skills/commands/agents cannot land as unregistered file drops.