feat(api): add backend with kanban, docs store, and orchestrator

Introduce the @void-nav/api Hono + SQLite backend that powers the docs site:
a persisted implementation board (kanban), a custom documentation-pages store
with AI beautify, and an agentic orchestrator that runs `pi` agents per card.

The orchestrator spawns `pi --mode rpc` inside an isolated git worktree per run,
streams slim events over SSE, and lets the agent drive the board/docs via
token-gated internal endpoints (all SQLite writes stay in-process). Interrupted
runs are reconciled to "stopped" on boot.

Workspace wiring: root `dev:api`/`dev:web` scripts with `concurrently`, the
docs Vite `/api` proxy, and `.worktrees/` gitignore.

apps/api/.gitignore

@@ -0,0 +1,3 @@
+# Local SQLite database and runtime artifacts
+.data/
+dist/