fix(agent): include goal lifecycle tools in Cowork

[bobleer]

Summary

• Add get_goal, create_goal, and update_goal to the Cowork default tool set.
• Add regression coverage for the complete goal lifecycle tool bundle.

Fixes #2009

Type and Areas

Type: Bug fix

Areas: Rust core, Agent runtime assembly

Motivation / Impact

Cowork sessions can create a thread goal through the product UI, independently of the tools exposed to the model. Without update_goal in the default tool set, an active goal cannot be completed and the automatic continuation loop keeps running. Cowork now exposes the same complete goal lifecycle bundle as the shared coding modes.

Verification

• pnpm run fmt:rs
• cargo test -p bitfun-core --no-default-features --features agent-runtime --lib agentic::agents::definitions::modes::cowork::tests — 2 passed
• git diff --check

Reviewer Notes

This is a clean replacement for #2036, based on current main; that PR contains unrelated commits and is currently conflicting.

AI-assisted; fully tested for the changed Cowork default-tool behavior.

Checklist

• This PR is focused and does not include secrets, temporary prompts, generated scratch files, or unrelated artifacts.
• Relevant verification is recorded above, or skipped checks are explained.
• User-facing strings, docs, and locales are updated where applicable; no copy changes are required.