docs(architecture): phase 1 bootstrap — specs, 9 ADRs, OQ tracker

Architecture documentation structure per sdd_process phase 1:

- README index (doc table, ADR table, lifecycle), overview with crate
  map, dependency rules, and security invariants
- Component specs: storage, transport, http, ssh, alkgitd (all draft)
- ADRs 001-009: crate decomposition, front-door-blind core, V2-first
  protocol, pack pipeline (data::output generation / data::input
  ingestion), session substrate types, http adapter composition
  (proposed, OQ-01), ACL-before-advertisement, registry-resolved repo
  identity, bounded-resources budgets
- open-questions.md: OQ-01..08 with two deferred(scope), one
  deferred(unclear), door-type definitions, blocker tracker tasks in
  tasks/architecture/
- v1 ssh-door decision recorded: russh terminates wire SSH in alkgitd;
  alkcall channels stay the internal substrate (OQ-03 partially
  resolved)

Two review rounds (fresh-context subagent): 4 critical + 17 warnings
fixed in round one; zero critical + 4 warnings + 5 suggestions fixed in
round two. All ADR/OQ cross-references verified resolving.

tasks/architecture/oq-06-metadata-backing.md

@@ -0,0 +1,37 @@
+---
+id: architecture/oq-06-metadata-backing
+name: OQ-06 unblock — metadata-scale requirements arrive
+status: pending
+depends_on: []
+scope: narrow
+risk: trivial
+impact: component
+level: research
+tags: [external-trigger, deferred-oq]
+---
+
+## Description
+
+Tracker for OQ-06 (`deferred(scope)`): registry/metadata backing store.
+This task is not actionable work — it tracks whether the blocking
+condition has arrived: concrete metadata-scale requirements (repo count,
+metadata fields beyond id/root/visibility/ACL scope, alkcall-hub
+integration scope). When they exist, mark completed; OQ-06 transitions to
+`open`.
+
+## Work
+
+None by default. On trigger: set OQ-06 to `open` and schedule the backing
+decision session.
+
+## Verification
+
+- OQ-06 status matches `docs/architecture/open-questions.md`.
+
+## Out of scope
+
+- Choosing the backing store.
+
+## Summary
+
+> Filled on completion.