refactor: extract helpers from generateManagedAppImprovementTaskForType (#2836)

[atomantic]

Summary

generateManagedAppImprovementTaskForType() in server/services/cosTaskGenerator.js had grown to ~533 lines, mixing metadata assembly, override sanitization, precondition/perpetual-work gating, deterministic reconcile pre-steps, and watcher polling in one function. This extracts that logic into named helpers (mirroring existing siblings like checkStagePrecondition), leaving the main function a thin orchestrator.

New module-level helpers (all behavior-preserving, moved verbatim):

• buildImprovementTaskMetadata — base metadata + sanitized global/per-app override merge
• resolveTaskInputHook — buildTaskInput hook ({ skip } / { hookPrompt, hookOverride })
• resolveClaimWorkRouting — claim-work single-source router → promptTaskType
• applyPerpetualWorkGate — perpetual drain work-detector gate
• resolveBranchReconcileBlock / resolveIssueReconcileBlock — deterministic reconcile pre-steps
• resolveReferenceWatchBlock / resolvePrWatcherBlock — watcher polling + prompt-block injection

Each gate-checking helper returns a discriminated { skip } result the orchestrator forwards as its return null, and the block-producing helpers return an empty block for non-matching task types — so dispatch ordering, park/convergence semantics, cadence recording, and every early-return path are unchanged.

Test plan

• cd server && npm test -- cosTaskGenerator — 63 pass (source-pinned wiring guards for claim-work routing, {reviewers} interpolation, swarm block, model-literal, etc. all still green)
• cd server && npm test -- cosTask cos.test taskSchedule — 510 pass across 8 suites
• Runtime import smoke-check confirms the module loads and all public exports remain functions

Closes #2836