docs(v2/db): add database architecture reference

Split into three files linked from CLAUDE.md's v2 docs index:

- v2-db.md — overview: three-DB model, cross-mount rules, central-vs-session
  decision, design patterns, readers/writers map.
- v2-db-central.md — every table in data/v2.db plus migration history.
- v2-db-session.md — per-session inbound.db / outbound.db schemas, session
  folder layout, seq parity invariant, lazy schema evolution.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

CLAUDE.md

@@ -138,6 +138,9 @@ This project uses pnpm with `minimumReleaseAge: 4320` (3 days) in `pnpm-workspac
 |-----|---------|
 | [docs/v2-architecture-draft.md](docs/v2-architecture-draft.md) | Full architecture writeup |
 | [docs/v2-api-details.md](docs/v2-api-details.md) | Host API + DB schema details |
+| [docs/v2-db.md](docs/v2-db.md) | DB architecture overview: three-DB model, cross-mount rules, readers/writers map |
+| [docs/v2-db-central.md](docs/v2-db-central.md) | Central DB (`data/v2.db`) — every table + migration system |
+| [docs/v2-db-session.md](docs/v2-db-session.md) | Per-session `inbound.db` + `outbound.db` schemas + seq parity |
 | [docs/v2-agent-runner-details.md](docs/v2-agent-runner-details.md) | Agent-runner internals + MCP tool interface |
 | [docs/v2-isolation-model.md](docs/v2-isolation-model.md) | Three-level channel isolation model |
 | [docs/v2-setup-wiring.md](docs/v2-setup-wiring.md) | What's wired, what's open in the setup flow |