chore(release): v1.2.1

[Kendrick-Song]

Summary

Bump everos to 1.2.1 and finalize the CHANGELOG.

The bulk of this release is PR #361 (embed/rerank soft dependency); the rest is docs polish, observability tweaks, and CI plumbing.

Highlights

• [embedding] and [rerank] are now soft runtime dependencies — EverOS boots with only [llm] configured (Tier 1). Missing providers return HTTP 422 CAPABILITY_UNAVAILABLE at the endpoint level instead of aborting startup.
• everos cascade backfill CLI — three-phase interactive backfill (vectors → clusters → skills) for upgrading Tier-1 rows to Tier-2.
• LanceDB schema v2 — six business tables now allow vector NULL. Migration runs once on first startup under a cross-process file lock.
• MemoryRoot.default() → .resolve() — deprecation alias retained; will be removed in a future major.
• Uncalibrated recall scores — KEYWORD / single-route VECTOR now report recall_top_score_raw; recall_top_score is reserved for calibrated methods.
• Docs use /api/v2 — /api/v1 remains as a legacy alias.
• 1.1.4 backport fixes merged into main (fix: backfill missing 1.1.4 fixes into main #366).
• PyPI Trusted Publishing workflow (ci(release): PyPI Trusted Publishing workflow + /release skill #358) + /release skill.

Full details in CHANGELOG.md.

Release flow

After this PR merges: tag v1.2.1 on main → the tag-triggered release.yml builds, smoke-tests, and publishes via OIDC behind the release environment's manual-approval gate.

Test plan

• make lint — ruff + import-linter + assets + deprecated-names + github-docs + datetime + openapi drift (all pass)
• docs/openapi.json regenerated to match version = "1.2.1"
• uv.lock synced
• Full CI green on this PR

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