Native deep-review engine port — strategy + scaffold

[JSONbored]

Part of #983.

Context

The current state is a NATIVE re-port, not a vendored engine: gittensory re-ported only the unified comment by hand into its own strict tsconfig and drizzle/D1 idioms (src/review/unified-comment*.ts, zero reviewbot imports). reviewbot's docs/CONVERGENCE.md vendor/embed blueprint (vendor under vendor/reviewbot with a relaxed tsconfig to dodge gittensory's 4 extra strict flags) did NOT happen and is moot. Every remaining reviewbot capability must be re-ported the same hand-written way. reviewbot's reviewTarget compute seam (engine.ts:16/31, the COMPUTE half of core/runtime.ts) is the reference for the deep-review engine; its act path (auto-merge/close/approve/label, runtime.ts ~635-1051) is DROPPED because gittensory's autonomy/action-executor/approval-queue already exceed it.

Deliverables

• Adopt the per-capability NATIVE re-port strategy as the committed end-state: zero reviewbot dependency / zero vendored copy when convergence finishes (the comment is already native; everything else follows the same pattern).
• Define the flag-gated default-OFF pattern (one settings flag per capability, default OFF, soak, then default ON) so processGitHubWebhook is behavior-identical when every flag is off.
• Define the gittensory module layout for the ported deep-review engine (where the compute seam lives, how it returns decision/reviews[]/readiness, how those feed gittensory's autonomy/executor rather than acting directly).
• Scaffold the deep-review module + flag + a placeholder test so subsequent capability ports (#gate, #safety, #RAG, #ops) attach to a known seam.
• Confirm reviewbot's act path (core/runtime.ts act half) is explicitly NOT ported — gittensory's agent-action-executor / agent-approval-queue / autonomy stay authoritative; only the COMPUTE half of reviewTarget is consumed.

Acceptance criteria

• A documented, agreed native-port strategy + gittensory module layout exists; the vendor/tsconfig/REVIEWBOT_ENGINE_ENABLED quarantine path is explicitly abandoned.
• The deep-review scaffold compiles under gittensory's strict tsconfig, is flag-gated default-OFF, and leaves processGitHubWebhook byte-identical when off.

Notes

• Milestone: M1
• Depends on: Finish the unified PR comment (close out PR #1007) #1011
• High-rebase-collision risk: codex churns processors.ts, comments.ts, and signals/engine.ts across other gittensory worktrees. Keep convergence edits minimal + flag-gated and rebase onto main before in-flight codex PRs touching those files settle.
• reviewTarget returns reviews[]/readiness as additive optional fields; if/when cached, readiness/CI must be recomputed FRESH at post time, never from the cached decision, or a later CI webhook re-renders a stale CI chip.