Separate implemented design from active proposals

[fazpu]

Summary

• replace the mixed improvement-proposals document with a binding, cold-readable design for the reliability and operations features that shipped in 0.3.0–0.6.0
• move only the still-relevant unimplemented work into design/proposals/: P1.2, the P2.1 remainder, the P2.4 remainder, and deferred deeper depth-first child chains
• update the decision log, working agreement, Skill, canonical docs, and public docs so their single-worker, recovery, telemetry, cost, and session-layout claims match the implementation
• record rejected or withdrawn ideas as deliberate boundaries instead of leaving them in the active proposal queue

Verification

• uv run pytest — 216 passed
• uv run ruff check .
• npm run build in website/ — 15 static routes; Pagefind indexed 12 docs pages
• relative Markdown links validated
• stale references to the deleted design path checked