Update root AGENTS.md and styleguide.md to reflect treefort rebrand

[bjester]

Observed Behavior

While template files in .treefort/templates/ were updated to reflect the new treefort project name, the root project's own AGENTS.md and .gemini/styleguide.md still refer to gh-worktree and src/gh_worktree/.

Expected Behavior

The root AGENTS.md and .gemini/styleguide.md should consistently reference treefort and src/treefort/ to align with the renamed repository and source directory structure.

Steps to Reproduce

1. Inspect AGENTS.md in repository root.
2. Note lines referring to gh-worktree and src/gh_worktree/.
3. Inspect .gemini/styleguide.md line 4.
4. Note references to gh-worktree.

Minimal Reproduction Environment / Conditions

• Version / Branch / Commit: release-v0.5.x@6d07c0c
• Files: AGENTS.md, .gemini/styleguide.md

Deliverables & Testing Requirements

• Update AGENTS.md to reference treefort and src/treefort/.
• Update .gemini/styleguide.md to reference treefort.

Technical Pointers & Root Cause Clues

• Suspected Components: AGENTS.md, .gemini/styleguide.md
• Related Commits: 226762e (Rename project to treefort)

Notes & Edge Cases

• Ensures AI developer instructions and developer guides stay consistent across worktrees.

Metadata

Severity / Priority: P3 (Low)
Area: documentation

AI Usage

Drafted with Gemini following the DEEP framework based on code review findings against release-v0.5.x. Reproduction conditions and code references were verified in the codebase.