[mcp] get/set AI budget limits via MCP — account ceilings, per-tree defaults, Loop iteration cap #476

Depends on the config resolver (backend ticket).

Scope

Expose the budget config through MCP so it can be read and set without the UI:

• get_budget_limits — returns the effective ceilings (token + charged), per-tree defaults, Loop cap, current rolling-24h consumption, and distance-to-limit.
• set_budget_limits — patch semantics; clamped to the backend max; owner-scoped (and an admin variant for the platform default).

Follow the existing MCP tool conventions (scopes: this is a write; no dry_run needed for a settings write, but validate + echo the resolved effective values). Keep it out of any read-only mode gate appropriately.

Why

Parity with the UI — the user asked for both. Also lets an operator raise a ceiling from an MCP client mid-incident instead of editing a constant and redeploying.