fix(proxy): detect git branch per-request to fix stale branch header

[AntonYeromin]

Summary

When a user starts CodeMie CLI on main and then checks out a new branch during the session (e.g., asks the LLM to create/checkout a branch), all subsequent LLM requests were still sending X-CodeMie-Branch: main because the branch was captured once at session start.

This caused token usage to be attributed to main in analytics instead of the newly checked-out branch.

Changes

• HeaderInjectionPlugin.onRequest(): re-detect current git branch via detectGitBranch(process.cwd()) on every LLM request instead of using the stale config.branch from session start
• Falls back to config.branch (session-start value) if git detection fails (non-git dir, detached HEAD, timeout)

Impact

Before: all LLM calls after a branch checkout → branch=main in analytics
After: LLM calls reflect the actual current branch at the time of the request

Checklist

• Self-reviewed
• Manual testing performed