fix(linearbot): stop working an issue that is taken back

[0xAlcibiades]

Closes #1559

Change

parseIssueReleaseWebhook is the mirror of parseIssueAssignmentWebhook, and
has to be a separate parser because that one returns null the moment the bot
is no longer on the issue — the payload that says "stop" is exactly the one it
discards.

A release is the bot being dropped as assignee and as delegate, with
updatedFrom naming the bot as the previous value. That last part is load
bearing: "not assigned to the bot" describes almost every issue in a workspace,
so without it every unrelated edit anywhere would qualify.

Two cases deliberately do not count:

• Losing one field while still holding the other. A delegate that is still
  the assignee is still meant to be working.
• The bot releasing the issue itself. Handing the issue back at the end of a
  turn is how an agent signs off; acting on it would interrupt the turn that
  just did the work.

Handling splits on whether the turn started

A handoff turn does not begin the instant its webhook lands — it waits its place
among concurrent handoffs, then spends seconds fetching issue context and
spawning a sandbox. So pendingAssignments tracks, per thread, whether the turn
has started, because the two cases need different things:

• Not started. Dropped where it stands. Nothing has been posted and no
  sandbox exists, so never starting is the whole of the cleanup.
• Started. Interrupted through POST /api/session/{thread_key}/interrupt,
  which ends the execution and lets the sandbox fall to the normal idle reclaim
  instead of holding a slot against --session-sandbox-running-limit for the
  rest of a turn nobody wants.

A thread this process has no record of is interrupted too. A turn that outlived
a restart is exactly the case where the issue looks busy and no local state
explains it, and interrupt reports interrupted: false harmlessly when there
is nothing running.

The map is per process, like the work it tracks. A restart loses the entries,
which costs nothing: it also loses the turns.

What this does not do

It does not touch the issue's status. Whoever took the issue back decides where
it belongs, and moving it here would fight them.

Testing

Nine parser tests: dropped as delegate; dropped as assignee; handed to someone
else outright; still held by the other field (no fire); an unrelated edit on
someone else's issue (no fire); no updatedFrom at all (no fire); the bot
releasing it itself (no fire); a create (no fire); and updatedFrom nested
under data.

bun test in services/linearbot: 117 pass. bun run check:types clean.

[0xAlcibiades]

Folded into #1537 — the release path and the start stagger are the same handoff path, and reviewing them apart meant reading the same call site twice.

[0xAlcibiades]

Reopened — this and #1537 touch the same call site but are separate fixes, so they are easier to review apart. #1537 lands first; this one rebases onto it.