tree test: the "too deep subtree" case never exercises maxDepth #678

What

#635: move_bot refuses cycles, overflow, duplicates and a too deep subtree (plugins/tree/test/tree.test.js) promises four refusals in its name and tests three: a descendant target (/descendant/), moving onto itself (/its own parent/), an existing duplicate name and a duplicate Bot name. maxChildren overflow has its own test. maxDepth is never exercised — a move_bot that silently dropped the depth check would keep the whole suite green.

The behaviour itself is right today; Reviewer verified it by hand: root → A → B → C → D, then moving a leaf E under D returns limit reached: E would end at depth 6 (maxDepth=5). That probe is the missing test.

Acceptance

1. A test that builds a chain deep enough for the move to exceed maxDepth (the probe above, or setup({ maxDepth: 3 }) with the same shape) and asserts move_bot refuses with /limit reached: .*would end at depth \d+ \(maxDepth=\d+\)/.
2. Either fold it into the existing "…too deep subtree" test so the name stops lying, or rename that test and add the depth case as its own — pick one, not both.
3. plugins/tree suite green at the tip, and the new case fails when the depth check in move_bot is removed (state that in the PR body as the red evidence).

Scope: plugins/tree/test/tree.test.js only; no source change unless the red run shows the check missing.

Written-by: Triage