feat: $skill mentions — full backend + frontend + CLI

[praxstack]

Summary

Full-stack $skill mention system: $skillName syntax for referencing skills in prompts.

Changes (21 files, +4820/-1425)

Backend: skill.service.ts, session_skills migration, API routes, prompt injection, message-v2 types
Frontend: autocomplete popover, inline pills, badge strip, global-sync, skill parser + 18 tests
CLI: TUI prompt skill autocomplete
SDK: auto-generated types + client

CEO Review (SELECTIVE EXPANSION)

• ✅ Contamination cleanup (.opencode/agent/docs.md removed)
• ✅ Max 5 skills per session limit + dedup check
• ✅ Fuzzy search (existing useFilteredList infrastructure)
• Deferred: skill usage analytics, content preview tooltip

Review Notes

• Modified files contain prax-dev snapshots (upstream pattern divergence expected)
• Parser: 18 unit tests covering edge cases
• High upstream conflict risk on prompt.ts, session.ts, message-v2.ts

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[github-actions]

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