fix(coding-agent): preserve context lifecycle and last answers

[fitchmultz]

Summary

Preserve Pi's context-window behavior while fixing four lifecycle bugs:

• Reserve an idle custom wakeup before awaiting startup guidance, so a second wakeup queues instead of racing the first.
• Honor late Agent cancellation before committing or announcing a fresh context window.
• Complete the early compaction phase on no-op and cancelled paths, releasing accepted input through the existing queue.
• Keep the last answer available after rollover and resume, using only the selected branch and preserving active-message precedence and empty-answer rules.

Also remove the duplicate message-token estimator and document the empty startup-hook prompt for idle custom wakeups. No new queues, locks, cancellation controllers, transcript fields or dependency changes.

Verification

• Exact final regression tests against the original implementation: 11 expected failures and 10 passing controls. Fixed implementation: all 21 selected cases pass.
• Six changed test files: 108 tests pass.
• Targeted affected and neighboring suites: 251 pass, 23 existing real-provider-gated skips.
• npm run check passes, including types, dependency/generated-lock checks and browser bundle smoke.
• Native Enter/Escape/queue methods run against the real session and faux provider; rendering is stubbed. File-backed SDK resume is exercised. No live clipboard, RPC process or real-provider run is claimed.

The unbuilt-worktree tests use Vite's native resolve.tsconfigPaths option in a temporary configuration to resolve existing workspace source aliases. No test configuration workaround is added to the repository.

Scope

Nine files; production agent-session.ts changes are +61/-55. Responses transport work and the Undici update are separate changes. This PR does not merge, release or activate anything in the live installation.