fix: load under bb >= 0.40 (presentation.label + non-recursive tool schema)

[DianaMak-Di]

The plugin currently fails to load on bb 0.40.0 (plugin SDK >= 0.4.16). Installing from main succeeds, but the plugin never starts, so the todo tool is never registered.

Two independent breaks, one after the other:

1. experimental_statusLabels was removed

registerTool: "experimental_statusLabels" was folded into "presentation" (labels) in SDK 0.4.16 (tool "todo")

The host rejects the field outright (RENAMED_AGENT_TOOL_FIELDS). Moved to the current shape:

presentation: {
  label: { pending: "Updating todos", completed: "Updated todos" },
},

2. Recursive tool parameter schema

With the above fixed, the next load attempt fails on:

tool "todo" parameters contains recursive JSON Schema $ref "#/$defs/__schema0"

nestedTodoInputSchema was a z.lazy() self-reference through children, which emits a recursive $ref. bb validates tool parameters with assertNoRecursiveJsonSchemaReferences and refuses any cycle.

This PR unrolls the nesting to a fixed MAX_NESTED_TODO_DEPTH (5) via a buildNestedTodoSchema(depth) helper. The emitted schema is finite and well under the 128 KiB parameter cap (PLUGIN_AGENT_TOOL_PARAMETERS_MAX_BYTES).

Open question

Fix 2 is a behaviour change — children was previously unbounded in depth and is now capped at 5 levels. I picked 5 as a practical default, but it is your call: happy to change the constant, or to take a different approach entirely (e.g. keeping the recursive Zod type for internal validation and handing the host a depth-limited schema separately) if you would rather not cap the public contract.

Verification

• npm test passes
• Verified end to end on bb 0.40.0: bb plugin reload todo reports running, the composer todo card and header badge render, nested/dependsOn/requires items behave as before, and bb todo show --thread … prints the tree
• dist/ is untracked, so this touches source only

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