Adds an `AgentIdentityRegistry` that stores `agent_name -> canonical_uuid`
at `<home_dir>/agent_identities.toml`, written atomically (write to
`.tmp.<pid>.<seq>.<nanos>`, fsync, rename). The registry is consulted on
every top-level spawn — if a binding exists for the agent's name, that
UUID is reused verbatim; otherwise the historical
`AgentId::from_name` derivation is used and the result registered as
the canonical UUID. Hand / child agents (parent.is_some()) keep their
existing UUID-v5-from-hand-id semantics.

The kernel grows a sibling `kill_agent_with_purge(id, purge: bool)`:
the existing `kill_agent` is now a thin shim that preserves the
binding (so internal lifecycle resets — panic restart, manifest
reload, hot reload — never silently orphan history). The destructive
purge path is reachable only through the explicit confirmation flow.

API surface (refs #4614):
- `DELETE /api/agents/{id}` now requires `?confirm=true`. Without it
  the request is rejected with `409 Conflict` and the data-loss
  warning text from the issue ("permanently remove its canonical
  UUID and all associated memories and sessions"). With confirm,
  the agent is killed AND the canonical UUID is purged.
- `GET /api/agents/identities` lists the registry contents (RFC3339
  `created_at` per row).
- `POST /api/agents/identities/{name}/reset?confirm=true` drops the
  binding without killing a running agent (next spawn re-derives a
  fresh UUID). Same 409-without-confirm gate.

CLI (refs #4614):
- `librefang agent kill <id>` auto-sends `?confirm=true` (the verb is
  the user's confirmation).
- `librefang agent delete <name> [--yes]` resolves name -> canonical
  UUID via the new endpoint, prompts `[y/N]` with the warning text,
  and issues the confirmed DELETE.
- `librefang agent reset-uuid <name> [--yes]` purges the binding via
  the new reset endpoint without touching the running agent.
- `librefang agent merge-history <name> --from <old-uuid>` is a stub
  that prints the deferral rationale and exits 2.

Dashboard: `deleteAgent()` now sends `?confirm=true` so the modal
flow continues to work end-to-end.

Tests:
- Unit (in `agent_identity_registry.rs`): round-trip read/write,
  first-register-wins, purge persists, malformed file leaves the
  on-disk file alone, in-memory persist is no-op, list ordering is
  deterministic.
- Integration (`crates/librefang-api/tests/agent_identity_registry_test.rs`,
  6 cases): canonical UUID is registered on spawn; bare DELETE
  returns 409 + warning + preserves identity; confirmed DELETE
  purges + respawn re-registers; malformed UUID still 400; list
  endpoint surfaces entries; reset endpoint gates on confirm + 404
  on missing.
- Existing `api_integration_test`, `load_test`, and
  `agents_routes_integration` callers updated to send
  `?confirm=true` (a `delete_confirmed` helper was added in the
  latter so future tests don't have to open-code the URL).

Deferred to a follow-up (documented in CLI long_about):
- `librefang agent merge-history` cross-table reassignment. Moving
  sessions / memories / events / kv_store / entities / relations /
  usage_events / canonical_sessions / prompt_experiments /
  audit_entries / approval_audit + the proactive memory store from
  one UUID to another under a single transaction with rollback is
  deep memory-substrate surgery and warrants its own PR with
  data-integrity testing.

Closes #4614