D285 — Tool descriptions have a size budget, enforced by a test

Amends D65.

Decision. The agent-profile tools/list of one unprefixed mount must stay within 22 KiB serialized, and no tool description may exceed 600 characters. TestServer_ToolDescriptionBudget fails the build otherwise and prints the five heaviest tools. Only description strings are shortened: tool names, property names, types, required and enums do not change.

Why. D65 fixed the agent profile at 17 tools (about 1.9k tokens of schemas); it has grown past 35 and the listing had reached about 41 KB, of which descriptions and property descriptions were most. Every client pays that text on every round-trip, in both mount modes, so routing (D187, D288) removes the duplication but not the weight of the single copy. A budget in a test is the only form that survives the next tool: prose in a convention is read once, a failing build is read every time.

Alternatives rejected. - Move more tools to advancedToolNames: a descriptor-bound host cannot call a tool tools/list does not advertise (D123), so hiding a tool removes it for that host. - Trim only the tool descriptions, leave the schemas: property descriptions were as heavy as the tool descriptions, and the fixed structure is already about half of the budget. - A convention without a test: it decays one helpful sentence at a time.

Consequences. A description keeps what the tool does, when to use it instead of its neighbour and every rule needed to call it correctly; rationale, D<n> references and response-field listings go to docs/control-plane.md §Tool descriptions and their size budget, which stays the full reference. A new tool that breaks the budget must be written shorter, or the budget raised in a decision of its own. Enforced limits (D161) stay interpolated from their constants. The budget is tight (a few hundred bytes of headroom): adding a tool means shortening a neighbour.