Retire Kimaki session helper for native cwd routing

[chubes4]

Summary

• remove the legacy datamachine-kimaki-session helper that wrote directly to Kimaki session/worktree SQLite tables
• route existing-worktree handoff through native kimaki send --cwd <path> by keeping datamachine-kimaki as the thin send-flag adapter
• clean up stale installed helper files on upgrade and update verification/docs/tests for native cwd routing

Closes #140.

Verification

• bash -n bridges/kimaki.sh bridges/kimaki/bin/datamachine-kimaki tests/kimaki-native-cwd-routing.sh tests/kimaki-no-sqlite-session-writes.sh tests/datamachine-kimaki-adapter.sh
• ./tests/datamachine-kimaki-adapter.sh
• ./tests/kimaki-native-cwd-routing.sh
• ./tests/kimaki-no-sqlite-session-writes.sh
• ./tests/bridge-render.sh
• node tests/effective-prompt/run.mjs
• ./tests/opencode-wrapper-removal.sh

Verification caveat

• ./tests/runtime-signature.sh currently fails on this macOS shell with awk: newline in string before reaching this change's code path.

AI assistance

• AI assistance: Yes
• Tool(s): OpenCode (GPT-5.5)
• Used for: Drafted and verified the helper removal, native cwd routing tests, docs, and PR description for Chris to review.

[chubes4]

Superseded by integration PR #162, which combines the Kimaki 0.13 cleanup work and resolves the overlapping bridge/test conflicts.