feat(webui): unified session/lease/lock/worktree inventory API (Closes #636)

Co-Authored-By: Claude Opus 4.8 (1M context) <[email protected]>

docs/webui-local-dev.md

@@ -212,6 +212,53 @@ health, workflow/schema SHA-256 hashes, and stale-runtime warnings when the
 checkout is behind merged safety-gate changes. Restart guidance links to #420;
 no tokens or MCP restart actions are exposed.
 
+## Inventory API (#636)
+
+`GET /api/v1/inventory` returns one versioned, read-only snapshot that unifies
+what the lease (#433), worktree (#432), and runtime (#430) MVP views each show
+separately, so traffic-control and recovery consumers read the same source.
+`GET /api/v1/inventory/{section}` returns a single section under the identical
+schema (`sessions`, `leases`, `locks`, `worktrees`, `namespaces`); an unknown
+section is a `404` with `error: unknown_section`. Both routes are `GET`-only.
+
+Each section carries its own `status` (`ok` / `degraded` / `unavailable`), a
+`reason` when not `ok`, and a `scan_ms`. A subsystem that cannot be read
+degrades to a reasoned section; it never raises and never emits an empty list
+that would read as "nothing is there".
+
+### Field authority
+
+Every section names where its rows came from; authorities are never blended.
+
+| Section | Authority | Source |
+|---|---|---|
+| `sessions` | `control_plane_db` | #613 control-plane DB (`mode=ro`), authoritative for exclusive ownership (#600/#601) |
+| `leases` | `control_plane_db` | #613 control-plane DB; degrades if the `work_items` table is absent |
+| `locks` | `filesystem` | durable per-issue lock files (`issue_lock_store`) |
+| `worktrees` | `filesystem` | registered git worktrees via the #432 hygiene scanner |
+| `namespaces` | `filesystem` | the active profile serving this web process (others are not enumerable) |
+
+The payload restates this map under `field_authority` for machine consumers.
+
+### Ownership safety
+
+`ownership_authority_complete` is true only when every ownership-bearing section
+(`sessions`, `leases`, `locks`) read cleanly. While it is false, nothing is
+reported as unowned and no collision is asserted from a degraded source —
+absence of evidence is reported as absence of evidence, never as free work.
+
+`collisions` surfaces detectable conflicts, each with a `kind` and `severity`:
+`lock-without-worktree`, `duplicate-live-lock`, `live-lock-dead-owner` (unexpired
+lease, dead pid — a #753 recovery candidate that would read as live to a naive
+timestamp check), `stale-lock-dead-owner`, `expired-lock-live-owner` (the
+#635/#760 daemon-pid deadlock), `concurrent-active-lease`, `active-lease-past-expiry`,
+and `orphan-lease`. Collisions are emitted only from sections that read cleanly.
+
+The control-plane DB is opened through a `mode=ro` URI so a read never creates
+or migrates it; paths are collapsed against `$HOME`, URLs lose userinfo and
+query strings, and credential-shaped values are redacted at the boundary. Lease
+steal/release and worktree deletion are Phase 2+ and have no representation here.
+
 ## Tests
 
 ```bash