fix: make durable PR waits callback-driven

[JWThewes]

Closes #334

Important

This PR must not be merged until #333 has merged. After #333 merges, this branch must be rebased onto the latest main and the checks rerun before this PR is made ready for review.

Problem

PR-per-unit review used durable timer polling. One production-like execution exhausted Lambda's 3,000-operation limit while waiting for review, leaving process state active after the durable execution had failed.

The affected intent also demonstrated why repair must preserve state: one reviewed unit PR had merged remotely while local state lagged, and another unit's construction stages and draft PR were already complete and reusable.

Approach

• Replace repeated durable PR polling with one persisted callback per observed provider state.
• Add sparse DynamoDB maintenance partitions for active executions and parked PR waits.
• Reconcile GitHub/GitLab state on a one-minute schedule and wake only for merge, closure, branch movement, or queued authenticated feedback.
• Atomically claim callback delivery so scheduler and feedback wakeups cannot double-resume a run.
• Detect terminal durable executions before their configured local expiry without overwriting a concurrently completed process row.
• Reconcile provider truth during repair, validate merged reviewed-head ancestry, preserve successful stages and artifacts, and resume existing draft PRs through the current orchestrator alias.

Recovery verification

The deployed dev recovery was exercised against the failed execution that motivated #334:

• the prior durable execution was confirmed FAILED with ExecutionLimitExceeded;
• the merged reviewed head was confirmed reachable from the intent branch and the corresponding unit was marked merged;
• ten completed construction stage rows remained SUCCEEDED at attempt 0 and no artifacts were archived;
• the existing draft PR was reconciled in place, promoted to ready, and parked on exactly one persisted callback in the sparse PR_WAITS index.

The review PR was deliberately left unmerged.

Testing

• npm test - 117 files, 2,331 tests passed
• focused shared/orchestrator/intents suites - 44 files, 887 tests passed
• commit hook - formatting, lint, secret scan, production dependency audits, and 561 targeted tests passed
• affected intents and v2-orchestrator Lambda bundles built successfully
• terraform fmt -check -recursive terraform