fix(cli): let Agent Manager tool fields be null so strict providers can omit them

[marius-kilocode]

Fixes #13029.

The agent_manager tool advertises one flat object covering both starting sessions and managing existing ones. That flat shape is required: Anthropic rejects a tool whose input_schema has a top-level anyOf, oneOf, or allOf, and since the tool sorts to tools[0] whenever KILO_CLIENT=vscode, a rejected schema fails the entire request rather than just that tool call. #13197 tried a discriminated union and had to be reverted in #13203 for exactly this reason.

The cost of flattening is that providers enforcing strict structured outputs, notably the OpenAI Responses API, must supply a value for every advertised property. None of the fields accepted null, so a model starting a worktree session had no way to decline action and was forced to invent a value. The invented action then took precedence over mode and tasks, so the tool returned an overview of existing sessions and no session was created. Reproduced on gpt-5.6-sol, which sends:

{"mode":"worktree","versions":false,"tasks":[{"prompt":"write a haiku","name":"probe","branchName":"probe-branch","model":"","variant":""}],
 "action":"list","filter":null,"sessionID":"","prompt":"write a haiku","sectionID":null}

Every advertised field is now nullable, so null is how a model states that a field is not part of the operation it chose, and the per-field descriptions say when to use it. The runtime already resolved action: null to a start request, so only the advertised schema changes here. The five-branch Params union remains the authoritative validator and its behavior is unchanged.

Two incidental cleanups in the same schema: versions and filter were double-wrapped in Schema.optional, which emitted a redundant nested anyOf. Both are now flat. The tasks bounds of 1 to 20 are preserved through the nullable wrapper.

This does not reintroduce strict rejection of mixed payloads. A model that still sends a populated action alongside mode and tasks continues to be routed by action. Turning that case into an explicit error is worthwhile but is a separate behavior change, tracked in #13205 along with the systemic Anthropic normalization in ProviderTransform.schema from #12905.

Also adds test/kilocode/tool-schema-provider-compat.test.ts, which walks every advertised tool and asserts an object root with no top-level combinator. It lives outside any single tool's test file and is phrased as a cross-tool invariant on purpose: this break shipped twice, and the second time the per-tool assertion protecting it was rewritten in the same diff that broke the shape, so CI stayed green. Confirmed the guard fails against the reverted-away schema before relying on it.