feat(skills): flat skill versioning + UCB1 adaptive-bandit routing (Phase 4)

[QodeXcli]

Phase 4 of the proposed evolution architecture — flat, OS-agnostic skill versioning with UCB1 adaptive-bandit A/B routing.

Storage (no symlinks, no split-brain)

~/.qodex/skills/<id>/
  manifest.json        ← single source of truth (versions, champion/challenger, per-version stats)
  SKILL.v1.md  SKILL.v2.md …

Identical on Windows/macOS/Linux, no admin rights. Legacy single-file SKILL.md skills keep working unchanged (routedSkillBody falls back).

The algorithm (pure, tested)

src/skills/learning/skill-versioning.ts — the exact spec'd types + functions:

• routeSkillVersion — UCB1: an unsampled version is explored first; otherwise it balances exploit (champion) vs explore (challenger); a challenger whose success rate collapses loses traffic automatically. Conservative on ties.
• createNextVersion — adds a challenger (parent = champion) without touching any other version's state.
• recordVersionExecution / decideChampion — converge the test: promote a clearly-better challenger, retire a worse one.
• Robustness over the sketch: next tag is max+1 (not count+1), and a retired version is kept in history (marked retired) so tags never collide and its file stays referenced.

I/O + CLI

• versioned-store.ts (atomic manifest writes): routedSkillBody, ensureVersioned, addChallenger, recordOutcomeAndConverge.
• qodex skill versions <name> — shows champion/challenger, per-version success rate + tokens, and the routed pick.

Live-verified

Seeded v1 → added a v2 challenger → fed the bandit a strong champion + weak challenger → it auto-retired the bad challenger and kept routing the champion.

Tests

14 (UCB1 explore/exploit, tag robustness after retire, stat updates, promote/retire/keep-testing). ✅ typecheck · ✅ full suite (1212) · ✅ build.

Note: Phase 5 (escalating-cascade judge) is a separate PR. Wiring the per-run execution-stat recording into the live agent loop is the remaining integration step on top of this storage+algorithm layer.