docs: consolidate active plans

[lzehrung]

Summary

• finish the historical plan cleanup, leaving 8 current plans
• consolidate the two language-expansion plans into one one-language-per-PR backlog
• separate measurement work from speculative performance implementation
• consolidate route, mobile bridge, and dispatch proposals under one semantic-synthesizer contract
• replace the stale priority list with a complete live index

Removed as completed, redundant, or rejected

• affected-tests command plan: shipped in P1: add affected tests command #146/feat: recover affected tests and extension mapping #203
• extension-mapping plan: shipped in Add configurable language extension mapping #147/feat: recover affected tests and extension mapping #203
• agent test-plan generation: redundant with affected, candidate-test output, and the existing test-command template
• residual performance cleanup candidates: rejected because no measurement establishes their value
• non-Git directory-mtime fallback: rejected because its persistent-state correctness cost outweighs current evidence
• superseded language and synthesizer plan files after consolidation
• previously identified completed and superseded plans

Remaining decisions

1. 2026-07-03-03-shared-server-lifecycle.md: retain only if reusable HTTP MCP servers are a supported product workflow. Otherwise delete.
2. 2026-07-03-14-privacy-preserving-diagnostics.md: retain only if support cases need a shareable bundle beyond doctor --json. Otherwise defer or delete.

Verification

• npx prettier --check docs/plans
• roadmap index references every remaining plan exactly once; no missing or unindexed files
• no references remain to deleted or renamed plans
• npm run check (232 files, 2,709 tests passed, 19 skipped)