fix(agent): make goal lifecycle available to every primary agent

[bobleer]

Summary

• Make the complete thread-goal lifecycle available to every BitFun-runtime primary agent, including Claw, DeepResearch, Team, custom modes, and external-source primary projections.
• Treat get_goal, create_goal, and update_goal as a required primary-session bundle so legacy or partial mode overrides cannot strand an active goal; Task child agents remain scoped to their declared tools.
• Stop advertising or querying BitFun thread goals in ACP-owned sessions, whose execution loop cannot consume local BitFun tools.

Motivation / Impact

Follow-up to #2178 and #2009. The earlier fix covered Cowork only. A full audit found the same UI/tool mismatch in the Assistant workspace (Claw), DeepResearch, Team, custom primary agents, external-source primary agents, and user profiles that removed one of the lifecycle tools.

The composer can activate a goal independently of a model tool call. A primary agent must therefore always be able to inspect and finish that goal, while child agents must not gain an independent thread-goal lifecycle.

Verification

• cargo test --locked -p bitfun-agent-runtime --lib — 284 passed
• Focused bitfun-core tests for built-in modes, custom modes, external primary/subagent projection, mode-profile overrides, and primary/subagent runtime policy — all passed
• pnpm --dir src/web-ui run test:run src/flow_chat/session-drivers/useComposerCapabilities.test.ts — 2 passed
• pnpm run type-check:web
• pnpm run check:core-boundaries
• pnpm run check:repo-hygiene
• git diff --check

Reviewer Notes

ACP sessions intentionally do not receive BitFun-local goal tools. The renderer now hides the local goal affordance and skips backend goal refreshes there; an ACP-native slash command with the same text can still be sent to the ACP agent.

AI-assisted; fully tested for the changed primary-agent goal capability contract.