perf(activity-groups): derive groups without model control calls

[ewen-poch]

Problem

begin_activity_group is a model-controlled MCP call used only to name and delimit activity groups. It adds a tool schema, system/turn instructions, permission traffic, and an additional inference step even though the host already groups real tool activity and can provide fallback titles.

Proposed change

• Remove the Activity Group MCP server, process entry point, runtime registration, and Codex bridge tool.
• Remove its system prompt and per-turn reminder from every ACP framework.
• Remove the deleted tool schema from static context estimates.
• Keep existing host-derived grouping, fallback titles, persisted event parsing, and historical declared-title rendering.

Scope and non-goals

This removes an internal control-call component; it does not add a new architecture or state machine. There is no data-model or data-relationship migration. Existing tool activity remains grouped in the UI, but the synthetic model declaration/permission step disappears. Asynchronous semantic title generation, Notebook output reduction, skill injection, and connector-context optimization remain out of scope.

Acceptance criteria and validation

All final checks ran after the last material edit:

• Activity tool/context injection is absent across ACP and Settings paths -> npm test -- src/main/acp/context-usage-static-context.test.ts src/main/acp/runtime.test.ts src/main/settings/service.test.ts -> 527/527 passed.
• Node and renderer contracts remain valid -> npm run typecheck -> passed.
• Repository lint remains clean -> npm run lint -> 0 errors, 19 baseline warnings.
• Repository regression suite remains green -> npm test -> 669 files and 9,752 tests passed; 15 files and 184 tests skipped.

Two independent reviews found no standards or specification findings. Uncovered risk: token reduction is inferred from removing the extra model-controlled tool step and static context, not measured by an automated provider-level token benchmark.

Review focus

Please verify that every active producer of begin_activity_group is removed while historical activity-group events and host fallback rendering remain compatible.

[github-actions]

Codex Review

Verdict: mergeable

No actionable findings.

Summary: Static inspection found no concrete merge-blocking defect in the changed code.