docs: scaffold documentation information architecture (Phase 1)

Add the docs/ skeleton for the full docs implementation: getting-started,
architecture, guides, reference, and vision sections, with a nav/TOC home
page. Every page is a stub carrying its heading outline and pointers to the
source material to be migrated in Phase 2 (content).

docs/nips/, docs/spec/, and the loose design docs stay at their current
paths (55 references from code, tests, and migrations outside docs/);
reference/nips.md and reference/design-docs.md index them instead.

Root GitHub-convention files are untouched. ARCHITECTURE/NOSTR/TESTING/
RELEASING/VISION* remain in place this phase and will become pointer files
in Phase 2.

Co-authored-by: npub19zza5yyr075j4vvlg2drsh2xe0xy3dqxhfchlgtw8vryglrry3gq8q7tzk <2885da10837fa92ab19f429a385d46cbcc48b406ba717fa16e3b06447c632450@sprout-oss.stage.blox.sqprod.co>
Signed-off-by: npub19zza5yyr075j4vvlg2drsh2xe0xy3dqxhfchlgtw8vryglrry3gq8q7tzk <2885da10837fa92ab19f429a385d46cbcc48b406ba717fa16e3b06447c632450@sprout-oss.stage.blox.sqprod.co>

docs/reference/design-docs.md

@@ -0,0 +1,19 @@
+# Design Documents Index
+
+> **Status: stub — Phase 1 (structure).** Content to be written in Phase 2.
+>
+> **Scope:** Index of the loose design docs and formal specs (kept at their current paths — referenced by code/tests; do not move).
+
+## Outline
+
+- `docs/MCP_DRIVEN_HOOKS.md` — MCP-driven lifecycle hooks
+- `docs/bridge-channel-window.md` — bridge /query channel window (defers to NIP-CW)
+- `docs/git-on-object-storage.md` — git refs over object storage (formal spec, pairs with docs/spec/GitOnObjectStore.tla)
+- `docs/mesh-llm-local-build.md` — mesh LLM local build prereqs
+- `docs/multi-tenant-relay.md` — multi-tenant relay formal spec (pairs with docs/spec/MultiTenantRelay.tla, MultiTenantAuth.spthy)
+- `docs/multi-tenant-conformance.md` — multi-tenant conformance checklist
+
+## Source material
+
+- `docs/*.md` loose files
+- `docs/spec/` TLA+/Tamarin models