Retire Kimaki session helper for native cwd routing

[chubes4]

Summary

• Remove the datamachine-kimaki-session helper and its direct Kimaki SQLite writes.
• Keep the datamachine-kimaki adapter for dispatch/flag normalization, and remove any installed legacy session helper during Kimaki bridge sync.
• Update DMC handoff instructions and snapshots to use kimaki send --cwd.
• Fix the mu-plugin block rewrite tests for macOS awk/stat portability uncovered while running the shell suite.

Closes #148.

Verification

• ./tests/bridge-render.sh
• ./tests/datamachine-kimaki-adapter.sh
• node tests/effective-prompt/run.mjs
• fail=0; while IFS= read -r -d '' f; do if ! bash -n "$f"; then echo "FAIL: $f"; fail=1; fi; done < <(find . -type f -name '*.sh' -not -path './.git/*' -print0); exit "$fail"
• for f in tests/*.sh; do [ -x "$f" ] || continue; "$f"; done
• kimaki send --help verified native --cwd <path> is available.
• Inspected installed Kimaki source: send.ts emits cwd in the thread start marker; discord-bot.ts revalidates it and records ready worktree metadata via Kimaki's own thread_worktrees path.

AI assistance

• AI assistance: Yes
• Tool(s): OpenCode (GPT-5.5)
• Used for: Drafted the code/docs/test changes and ran local verification; Chris remains responsible for review and merge.

[chubes4]

Superseded by integration PR #162, which combines the Kimaki 0.13 cleanup work and resolves the overlapping bridge/test conflicts.