Write the deep-dive docs the AI pages promised

AI.md, AI_AUTONOMY.md and PULSE_PRO.md linked nine times into
docs/architecture/, which .gitignore marks as not for public release. The
targets were never missing, they were deliberately unpublished, so every one
of those links was dead for anybody but the maintainer.

Write the three promised documents against the code and publish them under
docs/ where the rest of the shipped set lives.

PATROL_ARCHITECTURE.md covers a run end to end. The interesting part is that
deterministic signal detection runs after the model, not before, so unmatched
signals catch what the model failed to file. Signal types, the thresholds
those signals derive from your own alert settings, and every condition in
Finding.ShouldInvestigate are documented from internal/ai/patrol_signals.go
and internal/ai/findings.go.

ASSISTANT_SAFETY.md documents the session state machine in
internal/ai/chat/fsm.go, its four states, the tool classification it runs on,
and its invariants. No write without a validated target, no second write
before the first is verified, no final answer about an unverified change, and
no attempt count that wears the gate down.

ASSISTANT_ARCHITECTURE.md covers the agentic loop around that machine, the
three-phase pipeline, why only execution parallelises and at what cap, the
read-before-write batch that must stay ordered, the look-before-asking gate
and its bound, and the stable error codes.

Also removed two older pointers into the same private directory, in API.md
and UPGRADE_v6.md, and the two references to ENTITLEMENT_MATRIX.md in
PULSE_PRO.md. That file exists locally and is a monetization document, so
publishing it is not a documentation decision.

Unresolvable intra-doc links are now 2 of 218, both internal release-control
documents deliberately withheld from the shipped set.

Contract-Neutral: documentation only

docs/API.md

@@ -492,8 +492,8 @@ Detailed storage usage per node and pool.
 
 ### Recovery (formerly Backups / Snapshots)
 Pulse v6 uses the recovery API to provide a platform-agnostic view of backup and snapshot artifacts.
-See `docs/architecture/RECOVERY_CONTRACT.md` for the provider-neutral contract
-(subjects, points, rollups, posture, and filter semantics).
+The endpoints below carry the provider-neutral contract covering subjects,
+points, rollups, posture, and filter semantics.
 
 - `GET /api/recovery/points`
   - Query params: