docs: remove obsolete implementation plans

[lzehrung]

Summary

• remove 15 completed implementation plans, one superseded plan, and the rejected check-only upgrade plan
• replace the stale July 3 priority list with a live status index for remaining work
• remove stale cross-links and repair retained relative documentation links

Verification

• retained plan relative-link check: 0 broken links
• removed-plan reference scan: 0 matches
• npx prettier --check "docs/plans/**/*.md"
• git diff --check -- docs/plans
• npm run check reached and passed production audit, lint, and formatting; build could not replace the in-use Windows native addon (EPERM), so the build/test phases did not complete

Scope

• branch is based on current origin/main
• unrelated worktree changes and the untracked July 31 freshness plan are excluded