feat: add skill usage analytics backend

[charlieviettq]

Summary

• add GET /api/v1/analytics/skills for skill usage analytics from tool_calls.skill_name
• add db.Store.GetAnalyticsSkills with SQLite, PostgreSQL, and DuckDB implementations
• include generated API client types for the new response shape

Compatibility

• additive API only
• no behavior changes to existing analytics endpoints
• empty skill names are ignored and response payload remains valid for empty data

Privacy/Data

• local analytics only; no data is uploaded
• aggregates existing tool_calls.skill_name values

Tests

• go test -tags "fts5,kit_posthog_disabled" ./internal/db ./internal/server ./internal/postgres ./internal/duckdb ./internal/backendcontract
• cd frontend && npm run check
• cd frontend && npm run generate:api
• git diff --check

Follow-ups

• parser improvements to populate skill_name for Codex/Cursor from SKILL.md reads
• frontend Top Skills analytics panel

[roborev-ci]

roborev: Combined Review (67450b7)

No Medium, High, or Critical findings were reported.

The only reported issue was Low severity and has been omitted per instructions.

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Panel: ci_default_security | Synthesis: codex, 5s | Members: codex_default (codex/default, done, 7m17s), codex_security (codex/security, done, 59s) | Total: 8m21s

[roborev-ci]

roborev: Combined Review (fed42d1)

Skill analytics changes have one Medium correctness issue; no security findings reported.

Medium

• internal/db/analytics.go:2254: last_used_at is populated from the session start/created timestamp (info.ts) instead of the timestamp of the message/tool call that used the skill. Long-running sessions will report stale skill usage times, and the same session-based timestamp pattern is mirrored in the PostgreSQL and DuckDB implementations.
  • Fix: Join tool_calls to messages and aggregate the latest message timestamp per session/skill, using that value for LastUsedAt and trend bucketing with a session timestamp fallback only when message timestamps are missing.

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Panel: ci_default_security | Synthesis: codex, 8s | Members: codex_default (codex/default, done, 5m25s), codex_security (codex/security, done, 16s) | Total: 5m49s