test(agentic): add queue/status/plan/register test files
Adds four new test files covering previously untested functions:
- queue_logic_test.go: countRunningByModel, drainQueue, Poke, StartRunner, DefaultBranch, LocalFs
- status_logic_test.go: ReadStatus/writeStatus field coverage + WorkspaceStatus JSON round-trip
- plan_logic_test.go: planPath sanitisation + readPlan/writePlan round-trips with phases
- register_test.go: Register (service discovery, core wiring, config loading), OnStartup, OnShutdown
All 56 new tests follow _Good/_Bad/_Ugly convention and pass with go test ./pkg/agentic/...
Co-Authored-By: Virgil <virgil@lethean.io>
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