fix(registry): allow ancestor↔descendant A2A so audit_summary can reach PM

[HongmingWang-Rabbit]

Summary

Discovered via deep workspace inspection during a maintenance cycle: every audit cron in molecule-dev was structurally broken. Security Auditor / UIUX Designer / QA Engineer would correctly try to deliver their hourly `audit_summary` to PM (per #50 + #75), the platform A2A proxy would reject it with 'access denied: workspaces cannot communicate per hierarchy', and the agent would silently fall back to delegating to its direct parent (Dev Lead). PM's category_routing dispatcher was never reached.

Live evidence (just now)

From Security Auditor's memory:
```
Conversation: Delegation results are ready. Review them and take appropriate action:

• [failed] Delegating to f3531ba4-d5b2-49c6-b312-950413706e45
  Error: access denied: workspaces cannot communicate per hierarchy
  ```
  (`f3531ba4...` is PM's workspace ID. Security Auditor's parent is Dev Lead.)

From Security Auditor's recent activity log:
```
2026-04-15T05:13:07 a2a_send ok Delegating to Backend Engineer
2026-04-15T05:12:25 a2a_send ok Delegating to Dev Lead
2026-04-15T03:51:15 a2a_send ok Delegating to Dev Lead
```
Every fall-back delegation went to Dev Lead instead of PM. PM's inbox shows zero `audit_summary` deliveries this entire session.

Root cause

`registry.CanCommunicate()` only allowed: self → self, siblings, root-level siblings, direct parent ↔ child. A grandchild → grandparent (Security Auditor → Dev Lead → PM) was rejected. The original design wanted strict hierarchy to prevent rogue horizontal A2A — but it also broke the leadership-chain pattern that any audit/escalation flow needs.

Fix

Generalise to ancestor ↔ descendant. Any workspace can talk to any ancestor (any depth) and any descendant (any depth). Direct parent/child remains a fast path that avoids the walk; sibling rules unchanged; cross-subtree A2A still rejected.

Implementation: `isAncestorOf(ancestorID, childID)` walks the parent chain in Go with a `maxAncestorWalk=32` safety cap so a malformed cycle in the workspaces table cannot loop forever. One DB lookup per step. For a typical 3-deep tree this adds 1-2 extra lookups beyond the direct-parent fast path. Could be optimised to a single recursive CTE later if profiling shows it matters.

Tests (13 total, all passing)

Test	Behaviour	Status
SameWorkspace	self ↔ self	unchanged
Siblings	same parent	unchanged
RootSiblings	both root	unchanged
ParentToChild	direct	unchanged (fast path)
ChildToParent	direct	unchanged (fast path)
Allowed_GrandparentToGrandchild	NEW — PM → BE	✅
Allowed_GrandchildToGrandparent	NEW — Security Auditor → PM (the actual bug)	✅
Allowed_DeepAncestor	NEW — 4-level chain	✅
Denied_UnrelatedAncestors	NEW — cross-subtree walks terminate denied	✅
Denied_DifferentParents	unrelated pairs (extended with walk-lookup mocks)	unchanged behaviour
Denied_CousinToRoot	extended walk mocks	unchanged behaviour
Denied_CallerNotFound	DB error fail-secure	unchanged
Denied_TargetNotFound	DB error fail-secure	unchanged

The previous `Denied_Grandchild` test (which asserted the wrong behaviour) was REPLACED.

Local verification

```
ok github.com/Molecule-AI/molecule-monorepo/platform/internal/registry 0.005s (13 tests)
```

Why platform-level

Per the 'platform-wide fixes are mine to ship' rule. Every org template hits the same broken audit-routing chain — fixing it at the platform benefits all users, not just molecule-dev. This unblocks #50 (PM dispatcher prompt) and #75 (category_routing) which both assumed audit_summary delivery worked.

Applied locally

Per the apply-locally rule, I'm rebuilding + force-recreating the platform container in parallel with this PR. Within ~2 min the live system will accept Security Auditor → PM, and the next hourly security audit (next fire 05:17 UTC, +5 min) will deliver its first audit_summary to PM successfully.

Related

• chore(template): PM system prompt — treat audit summaries as dispatch triggers, not FYIs #50 PM dispatcher prompt — receiver-side; was waiting for this
• feat(platform): generic category_routing replaces hardcoded audit dispatch (#51) #75 category_routing — declarative routing; was waiting for this
• Platform: scheduler stops firing crons silently — ran for hours then died #85 scheduler-died — discovered while debugging this same audit chain
• Platform: SYSTEMIC — every background goroutine lacks panic recovery + liveness probe (SaaS risk) #92 systemic supervisor — same investigation arc

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