fix(coding-agent): pause stale goals on direct input

[realsigridjin]

Summary

• pause an active durable goal as soon as newer interactive or RPC input arrives
• cancel pending monitor continuation synchronously and defer in-flight pauses until usage is fully accounted
• remove the unconditional userGrace continuation path while preserving extension-originated and unattended monitor continuation
• migrate legacy pi-goal state before session initialization without changing current-store precedence or tokenBudget round trips

Root cause

Direct input did not create another session. A stale active goal remained eligible for a hidden delayed continuation after the user had moved on, making the terminal appear to switch back to another task.

Behavior changes

• idle direct input accounts the existing idle window, then persists active -> paused before the new turn
• in-flight direct input preserves that turn's usage, then pauses at agent_end with a goal-id guard
• blocked and paused goals remain unchanged by ordinary input; blocked goals now require explicit /goal resume
• extension input never pauses a goal, and explicit resume re-arms one continuation
• current goal files always win over legacy files; only legacy migration removes obsolete budget metadata

Verification

• affected goal, wire, and regression suites: 9 files / 134 tests passed
• isolated migration commit: 3 files / 74 tests passed, plus full npm run check
• full npm run check passed on the final tree
• ordered root npm run build passed
• built and installed RPC plus native PTY/TUI stale-goal scenarios passed
• five independent review lanes passed after finding and fixing the current-store tokenBudget regression

Risk boundaries

• no authentication, dependency, lockfile, payment, or provider behavior changes
• timer tests use fake time and assert synchronous cancellation before the first awaited read
• changes are split into independently reviewable migration and input-precedence commits

Related to #447.

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Summary by cubic

Pause active goals on direct interactive/RPC input to prevent stale hidden continuations; remove the 60s “userGrace” timer and keep monitor-delayed continuation intact. Also migrate legacy pi-goal files into the current goal store at session start. Fixes #447.

• Bug Fixes

  • Idle input pauses active -> paused at the input seam; the new turn isn’t charged to the goal.
  • Input during a run defers the pause to agent_end after usage is accounted.
  • Cancels any pending monitor continuation immediately on input; extension-sourced input never pauses.
  • Blocked goals no longer auto-resume on user prompts; use /goal resume to reactivate.

• Migration

  • On session start, migrate from extensions/pi-goal/<threadId>.json to extensions/goal/<threadId>.json.
  • Drop legacy tokenBudget and normalize budgetLimited to active; existing current-store files always win.
  • Current-store tokenBudget metadata (if present) is preserved on read/write.

^{Written for commit 92af158. Summary will update on new commits.}