fix(core): exempt live-child locations from idle eviction

[KamitobiHaru]

Issue for this PR

Closes #51828

Type of change

• Bug fix
• New feature
• Refactor / code improvement
• Documentation

What does this PR do?

A background shell running a long job gets SIGTERM'd when LocationActivity
evicts its location after 60 minutes without session events. The service
process itself stays alive (same PID throughout); only the location-scoped
Shell/Pty graph is torn down, because locations.invalidate() closes the
scope-bound child-process handles.

Seen on 2026-09-27 (service 2.0.10): location services evicted directory=/data3/zsy/asr2report_pipeline at 16:24:48.527Z, torchrun
Received 15 death signal at 16:24:48.563Z, payload's own handler wrote
rc=143, signal=15. No service restart near the window, so this is the
inactivity sweeper, not the ensure() replacement path.

Why it happens: the sweep only exempts locations that own active
SessionExecution sessions. A backgrounded shell/pty emits no session
events and holds no execution claim, so its location looks idle and gets
invalidated out from under the live child.

Changes:

• New LocationRetention registry (packages/core/src/location-retention.ts,
  global node) tracking locations with live child processes, keyed by the
  same identity the sweep uses (LocationServiceMap.canonical +
  workspaceID via a shared keyOf, so pins can't silently miss).
• shell.ts / pty.ts: retain on create, release on settle/remove/teardown
  (per-command retained flag, no double-release). Teardown still kills
  running children and now logs them.
• location-activity.ts: expired entries with live children are touched
  (deadline deferred, no reference taken) and skipped; rest unchanged.
  Explicit invalidate still tears down immediately (registry is only read
  in the sweep). No task persistence.

How did you verify your code works?

• bun typecheck clean in packages/core.
• New tests: location-retention.test.ts (5 unit) and
  location-activity-retention.test.ts (2 TestClock sweep tests: a pinned
  location survives past TTL and evicts after release; explicit invalidate
  still tears down a pinned location without leaking the pin).
• Existing suites still pass: pty-session (7), location-activity (4),
  browser-idle (1, only needed the new node dep), session-shell /
  tool-shell / shell-retention (97 pass, 19 platform skips).
• Live server with production 60min TTL, idle for over an hour, real
  background shell (POST /api/shell, sleep 4000, DEBUG, isolated
  XDG_DATA_HOME): pinned 08:48:43Z, eviction deferred 09:49:22Z (first
  sweep after the 09:48:43Z deadline; the touch pushes the deadline an hour
  out so a single line is the whole story), running + process-alive polls
  at 09:48:58Z and 09:53:58Z, natural exit=0 with unpinned, idle eviction resumed at 09:55:23Z. Zero location services evicted, zero SIGTERM.
  Full logs attached: v2-server-monitor.log, v2-server-serve.log,
  v2-server-opencode.log, v2-local-verify.log.

Screenshots / recordings

N/A (no UI change).

Checklist

• I have tested my changes locally
• I have not included unrelated changes in this PR

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1. Open an issue describing the bug/feature (if one doesn't exist)
2. Add Fixes #<number> or Closes #<number> to this PR description

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[github-actions]

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