Fx module wiring and lifecycle cleanup

[yushan8]

Extract comparison logic into controller/comparer.go

Graph comparison logic moved out of getchangedtargets.go into a targetGraphComparer type. The controller calls newTargetGraphComparer(...).Compare(...) directly — no wrapper method.

Move per-request init to construction time in nativeOrchestrator

• Config parsing — config.Parse() was called every request. Params.ConfigFilePath replaced with Params.Config config.RepositoryConfigProvider, injected once.
• gitFactory nil fallback — moved to the constructor.
• graphRunner nil-check + inline bazel/runner creation — replaced with a GraphRunnerFactory func set at construction. defaultGraphRunnerFactory provides the bazel-backed implementation; tests inject their own.
• Metrics scope and counters — scope.SubScope("get_target_graph") and its counters are pre-created as struct fields instead of allocated per request.
• GetTargetGraph decomposed into prepareWorkspace (lease → checkout → apply → cleanup) and computeGraph (cache check → compute → store). The method itself is now metrics + prepare + compute.

Remove dead appCtx from orchestrator and repomanager

Both stored an application-lifetime context but never read it. Removed. Only the controller retains appCtx — it's actually used for linkRequestCtx and fire-and-forget cache writes.

Make all Params structs Fx-injectable

• controller.Params, orchestrator.Params, repomanager.Params all carry fx.In.
• Constructor signatures simplified from New(appCtx, Params) to New(Params).

Add module/module.go

fx.Module("tango") wires repomanager → orchestrator → controller. Documents what callers must supply.

Convert example/main.go to Fx

Uses fx.New(...).Run() with fx.Lifecycle hooks for dispatcher start/stop, temp directory cleanup, and signal-based shutdown.