feat(acp): add GOOSE_DISABLE_TOOL_CALL_SUMMARY to opt out of per-tool-call summaries

[ocervinka]

Motivation

The ACP server fires an auxiliary provider.complete_fast call for every tool
invocation. Its only purpose is to upgrade the synchronous fallback_title
(tool name + truncated args) into a model-generated short phrase shown in the
ACP UI's tool-call list.

For two real-world cases this auxiliary call is the difference between a run
that finishes and one that gets throttled mid-session:

1. Code Assist OAuth has a tight per-second cap. A single user turn that
   makes 4 tool calls fires 4 extra complete_fast requests on the same
   bucket, plus the existing session-naming call (when not opted out), and
   trips the 429.
2. Gemini API free tier allots ~250 requests/day for Flash. Per-tool-call
   summaries triple or quadruple the request count of any non-trivial agent
   loop and exhaust the quota long before the day ends.

Headless ACP integrations that don't surface the title at all still pay the
cost.

Change

Add GOOSE_DISABLE_TOOL_CALL_SUMMARY (env var / config.yaml key, bool,
default false). Behavior unchanged when unset. When set, the synchronous
fallback title remains visible to the client and the complete_fast round
trip is skipped.

This mirrors the existing GOOSE_DISABLE_SESSION_NAMING precedent line-for-line:

• registered via the same config_value! macro entry
• gated at the call site with the same Config::global().get_*().unwrap_or(false) shape
• documented in the same env-var reference table
• defaults preserve current behavior

Diff size

3 files, +17 lines, no deletions. No public API changes.

Test plan

Manual:

• With the var unset, every tool call still receives a ToolCallUpdate whose
  title is the model-generated phrase (existing behavior).
• With GOOSE_DISABLE_TOOL_CALL_SUMMARY=true, only the initial ToolCall
  notification is sent (with the synchronous fallback title); no follow-up
  ToolCallUpdate for the title; no provider.complete_fast call appears in
  provider logs.

No unit tests added — the existing GOOSE_DISABLE_SESSION_NAMING opt-out
ships without one and follows the same pattern. Happy to add a test if a
reviewer prefers; would likely involve a mock Provider asserting that
complete_fast is not invoked when the flag is set.

Checklist

• cargo fmt (formatting matches surrounding code)
• cargo clippy --all-targets -- -D warnings (will run via CI)
• cargo test (will run via CI)
• DCO sign-off
• Conventional Commits commit message

[DOsinga]

Clean, minimal change that follows the existing GOOSE_DISABLE_SESSION_NAMING pattern. I pushed a small cleanup commit removing the redundant inline comment (the config key name is self-documenting) and the example block in the env-var docs. LGTM.