fix(acp): use formatCompactTokenCount for subagent activity card toke…

[rohanshrma222]

Description

Fixes the subagent activity card in the ACP (VS Code extension) integration showing imprecise token counts. It previously computed Math.floor(tokenCount / 1000) and appended k, so 1–999 tokens displayed as 0k and 1500–1999 all displayed as 1k. Now it reuses the existing formatCompactTokenCount formatter from source/usage/format.ts (already used for the per-response usage indicator), so e.g. 1900 tokens correctly shows 1.9k tokens.

Closes #1133.

Type of Change

• Bug fix
• New feature
• Breaking change
• Documentation update

Changeset

• Added a changeset (pnpm changeset) describing this change for the changelog

Testing

Automated Tests

• New features include passing tests in .spec.ts/tsx files
• All existing tests pass (pnpm test:types, pnpm test:lint,ran locally, both clean)
• Tests cover both success and error scenarios

No new test added — this is a one-line formatting fix reusing an already-tested formatter (formatCompactTokenCount, covered by existing source/usage/*.spec.ts). Verified the fix's output directly with a throwaway script against the token counts from the issue (500 → 500, 1900 → 1.9k, 12345 → 12.3k, vs. the old 0k/1k/12k).

Manual Testing

• Tested with Ollama
• Tested with OpenRouter
• Tested with OpenAI-compatible API
• Tested MCP integration (if applicable)

This path only renders inside the ACP/VS Code extension subagent activity card and isn't reachable through the plain CLI providers above; verified via direct formatter comparison instead of a live ACP session (see Testing notes).

Checklist

• If this was for an open issue, I was assigned to it
• Code follows project style guidelines
• Self-review completed
• Documentation updated (if needed), not needed, no public API/doc surface changed
• No breaking changes (or clearly documented)
• Appropriate logging added using structured logging (see CONTRIBUTING.md (../CONTRIBUTING.md#logging)) — n/a, no new logging needed for this change