A deleted Bot keeps its open-turn records in the tree load tracker #732

What happens

plugins/tree/src/load.js keeps one record per open turn per Bot id, in a process-local Map (createLoadTracker). plugins/tree/src/tree.js:293 onBotRemoved() re-attaches the deleted Bot's children, repairs the organizer, refreshes prompts and runs adoption — it never drops that Bot's turn records. The tracker keeps the id forever, so snapshot() still counts a Bot that no longer exists, and every later read walks an entry nobody can ever end.

Harmless in size (bounded by how many Bots existed), but it is a leak on the one path that is supposed to dispose of a Bot, and it makes the tracker's own invariant ("every record belongs to a live Bot") false. Found by the #692 review (point 3) and left out of scope there.

Reproduce

node --test plugins/tree/test/load.test.js

with a case that starts a turn for b1, calls onBotRemoved({ botId: 'b1', parent: <root> }), then reads load.snapshot() — today it still lists b1.

What should happen

• createLoadTracker gains clear(id): removes that Bot's records and returns nothing; unknown id is a no-op. Pure, in the style of start/end/of.
• tree.onBotRemoved() calls it for the removed botId before or after the children re-attach (either order is fine; the tracker holds turn records only, never the tree shape).
• snapshot() and openFor() for that id read 0 once the Bot is gone.

Acceptance

• plugins/tree/test/load.test.js: clear(id) drops that Bot's records and leaves other Bots untouched; clear of an unknown id changes nothing; after clear, of(id) === 0 and openFor(id) === 0.
• a tree.js-level case (the existing plugins/tree/test/tree.test.js harness) asserting snapshot() no longer names the removed Bot after onBotRemoved.
• Red without the source change: delete the clear call from onBotRemoved → the tree-level case fails; delete the clear method → the tracker case fails on a missing function.
• plugins/tree suite green (node --test plugins/tree/test/*.test.js), changelog fragment added, and docs/subsystems/tree.md mentions that a removal drops the Bot's turn records.

Out of scope

No change to start/end/openFor semantics, no persistence, no behaviour change for live Bots.

Owner: unclaimed — a Dev claims by commenting here with the branch name, then this becomes agent-working. One issue, one pull request.

Written-by: Triage