feat(runtime): execute the transactional parent merge of child changesets

[OBenner]

Context — Phase B / PR-3 of "mutating subagents → safe"

Stacked on #320 (changeset export) — base branch is feat/subagent-changeset-export. After #320 merges, I'll retarget this PR to develop.

The merge plan stops being descriptive: after a run_subagents run whose plan requires a parent merge, the parent now applies each child's exported changeset transactionally.

What changes (generic_edit.py)

execute_subagent_changeset_merge — each mutating child is one transaction boundary:

1. Scope revalidation (defense-in-depth): every entry path must sit inside the child's declared write_scope.
2. Baseline verification: every entry's preimage must still match the live workspace (existing drift comparator). An unverifiable baseline refuses the merge exactly like real drift — no transactional rollback guarantee, no apply.
3. Atomic apply per child: postimages applied in order; a mid-apply failure restores that child's captured baseline in reverse — siblings unaffected.
4. One parent transaction per child: each applied changeset is recorded as a parent mutation snapshot with transaction id subagent_merge:<child-id> — the parent's existing rollback_transaction tool undoes exactly one child's merge.
5. Conflicts are never auto-applied: overlapping-scope children surface as conflicted_unresolved for explicit resolution (deterministic AutoMerger routing is PR-4).

run_subagents wiring: action data now carries merge_plan + merge_execution; the action stays ok only when every appliable changeset applied (applied/noop). Mutating delegation inside an open model-managed batch is refused up front (mutating_subagents_in_open_batch) before any child spawns.

Tests (6 new, 233 passed in the runtime file; 398 across the surface)

• End-to-end: parent (promoted direct-API session, bold) delegates to a REAL confined changeset child → the child's staged edit lands on the shared workspace via the parent, merge_execution.applied_result_ids == ["edit-a"], per-child transaction id recorded.
• Baseline drift → refused, workspace intact, no snapshot recorded.
• Mid-apply failure → first entry rolled back, nothing leaks.
• rollback_transaction("subagent_merge:edit-a") → file restored to baseline.
• Conflicted children → both conflicted_unresolved, nothing applied.
• Delegation inside an open parent batch → refused, factory never invoked.

🤖 Generated with Claude Code

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[sonarqubecloud]

Quality Gate passed

Issues
1 New issue
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud