[Epic]: Activate the dormant agent-plane capabilities — durable replay and open mode

### Summary

Slices 3 and 4 shipped **default-off**, and their parent epic (#353) was closed as *completed* while both of its declared hard gates stayed open. Two capabilities are fully built and inert in production, with no tracking parent.

### Intent

The house rule is *activate it or delete it*. Dormant code behind a default-off flag is the anti-pattern this epic exists to resolve — either these are turned on with their gates genuinely satisfied, or they are removed.

### Children

- **#363** — durable replay (`LCI_DURABLE_REPLAY` still defaults false)
- **#365** — autonomous `open` mode (`deliver_pr_open` still bails; no `CodePlatform::open_pull_request` anywhere)

### Acceptance Criteria

1. Each gate's items are **re-verified against `main` today**, and each is either struck with evidence or restated as still-open.
2. Each capability is enabled by default in production, or its code and flag are deleted. No third state.

### Out of Scope

- New agent-plane features.

### Technical Context

⚠️ **Both of #363's P1 gate items appear already satisfied on `main`, and every path it cites is wrong.** `add_pending_comment` is now `services/control-plane/src/db/pending_actions.rs` with an `ON CONFLICT (task_id, run_epoch, call_id) … DO NOTHING` and a test `add_pending_comment_dedups_on_call_id`; `upsert/fetch_durable_step` moved to `db/durable_step.rs` with round-trip coverage; `agent-clients/src/control_plane.rs` **no longer exists** as a file. A hard gate reads as blocking while its blockers are gone — re-verify before treating either as work.
Its two **P2s do remain open**: `content_hash` is still discarded on rehydrate, and `docs/jobs-and-lifecycle.md` still claims the runner clears the buffer at run start.

⚠️ **#365's TOCTOU AC is ambiguous** — there are **two** `resolve_write` implementations (`open-agent/src/workspace.rs` and `review-agent/src/tools/fs_safety.rs`) and it names only one. State which are in scope.

⚠️ Both gates were written against the **native** loop, which ADR-0097 demoted to legacy (reviews run on OpenCode since 2026-07-17). Parts of their task lists are stale even though the gates stand.

### AI Usage Declaration

Drafted by Claude Opus 5 during the 2026-09-01 triage sweep; the #363 gate items were verified against `origin/main` rather than the ticket text, which is how the drift surfaced. AI output is not truth; the owner verifies.