fix: filter git operations from session diffs

[R44VC0RP]

Summary

When git operations like pull, merge, or checkout are run (either through OpenCode or in a separate terminal), the changed files would inflate the session diff counts in the sidebar, showing thousands of lines changed that weren't actually edited by the agent.

Changes

• New FileTracking module (session/file-tracking.ts): Tracks files modified by git operations during a session
• Bash tool updates: Detects git worktree-modifying commands (pull, merge, checkout, rebase, etc.) and tracks affected files
• Step boundary tracking: Records the project's git HEAD at step-start and step-finish to detect external git operations
• Diff filtering: Excludes git-modified files from session diffs unless they were also explicitly edited by tools

How It Works

1. At step-start: Records the project's git HEAD
2. During execution: If bash runs a git op, tracks changed files immediately
3. At step-finish: Records the new git HEAD
4. When computing diffs:
   • Compares HEAD at step boundaries to find all git-modified files (catches external git ops)
   • Filters the final diff to exclude git-modified files
   • Still includes files if they were explicitly edited by edit/write/patch tools

Handles

• Git ops run through OpenCode's bash tool
• Git ops run in a separate terminal by the user
• User external edits (preserved - they show in snapshot diff but NOT in git-modified list)
• Files that are both pulled AND subsequently edited by tools (included)