test: add behavioral test coverage for cataractae dispatch paths

[MichielDean]

Summary

Adds behavioral tests for recently-shipped features with low Go-level coverage:

• Recirculation routing with explicit target (recirculate:<step>): verify the observe cycle routes to the agent-named step, overriding the step's on_recirculate route. Covers review→implement, implement→security, and unknown-target pass-through.
• Fork-mode dispatch integration: verify the full dispatch tick (dispatchRepo → DeliveryModeFork check → prepareDropletWorktree) creates the per-droplet worktree from upstream/main, not origin/main. Complements the unit-level prepareDropletWorktree fork tests.
• Liveness exit-no-outcome detection: verify a dead tmux session with no outcome triggers an exit_no_outcome event, releases the pool slot, and resets the droplet for re-dispatch. Also verifies the guard: a dead session WITH an outcome is not reset (observe owns that).
• Pool-reason dashboard display: verify fetchDashboardData populates the PoolReasons map for pooled droplets (single, multiple, none) and the GET /api/droplets/{id} endpoint includes pool_reason in the JSON response for pooled droplets, omits it for non-pooled, and omits it when no reason is recorded.
• Fork-mode delivery outcome routing: verify a fork-delivery step's pass outcome routes to terminal (done) — routing is delivery-mode agnostic.

Why

These paths had only unit-level or smoke-test coverage. The behavioral tests verify the actual dispatch and routing logic end-to-end through the scheduler tick, catching integration regressions that unit tests miss (e.g. the DeliveryModeFork → baseRemote selection in dispatchRepo was untested before this PR).

Test Impact

13 new tests. 2223 → 2236 passing. No production code changes.