Tree: stop_bot blocked by tree policy, stale prompts after reparent, createChild race (PR #2 review) #15

Context

The tree send middleware blocks some control messages, and reparenting after a deletion leaves stale state. Both were flagged in the PR #2 review and are still present at 7b806c6. Review items 1–3 (no ensureRoot, flybot.on crash, root reply to a non-tree Bot dropped) are fixed at 7b806c6 and are not tracked here. Related to #6.

Original feedback

• @hsliuustc0106 review (permalink), item 4: "src/host/tools.js:165 delivers the stop_bot notice with role: 'stopped' … the tree middleware can block a Main Bot from stopping a non-root tree Bot. Control messages (stopped, report) should not be policy-gated."
• Codex inline comment, Refresh prompts for reparented children (permalink), ds-bot/plugins/tree/src/tree.js L167 (P2), also in the review's P2 Tree list ("…and ignores maxChildren").
• P2 Tree: "createChild is check-then-act across an awaited create — parallel tool calls … exceed maxChildren/duplicate siblings."

Where (at 7b806c6)

• src/host/tools.js#L169 sends role: 'stopped'. In plugins/tree/src/middleware.js#L22-L26, the root is now allowed to reach outside the tree, but a non-tree sender to a non-root node still gets DENY_INTO, and the root to a grandchild still gets DENY_EDGE.
• plugins/tree/src/tree.js#L161-L168: onBotRemoved refreshes only up's prompt.
• plugins/tree/src/tree.js#L133: createChild

Proposed fix

• Exempt control roles (stopped, report) from the tree policy, or pass the role to the middleware and allow it.
• In onBotRemoved, call refreshPrompt(child.id) for each reparented child, and decide how to handle maxChildren overflow.
• Serialize createChild per parent (a mutex or an in-flight reservation).

Acceptance criteria

• A Main Bot can stop_bot any tree Bot.
• After deleting a middle node, each reparented child's prompt trace excludes the deleted node.
• N parallel create_child_bot calls never exceed maxChildren or create duplicate siblings.

Refs: PR #2 (merged), umbrella #3. Verified against Birfy/FlyBot flybot-plugins tip 7b806c6.