Count subagent model usage

[martian56]

Fixes #174.

Summary:

• Add successful subagent model replies to shared turn and session usage totals.
• Keep delegated usage visible through the existing turn usage notice and session status totals.
• Cover direct subagent usage and parallel dispatch usage with local provider tests.
• Bump the patch version and update the changelog.

Checks:

• rvpm test
• rvpm build
• rvpm fmt --check