Add configurable directory face mode for git status

[rain-64]

Summary

• Parent directories of changed files were hardcoded to treemacs-git-modified-face regardless of children's actual git status (e.g., a directory with only staged/added files showed as "modified")
• Introduces treemacs-git-directory-face-mode defcustom with three modes: modified (default, preserves existing behavior), severity (inherits highest-severity child status), and custom function
• Adds treemacs-git-directory-face-severity-list defcustom for configuring the severity hierarchy

The three modes

modified (default) — directories always get treemacs-git-modified-face. This is the existing behavior; no user configuration changes are needed.

severity — resolves directory faces using a severity hierarchy. When all children share the same status, the directory inherits that status. When children have mixed statuses, the highest-severity status wins. Default hierarchy (highest to lowest):

Rank	Status	Rationale
1	Conflict (U)	Blocks merges, must be resolved
2	Modified (M)	Unstaged changes, dirty working tree
3	Untracked (?)	Unknown to git, risk of being left out
4	Added (A)	Already staged — deliberate user action
5	Renamed (R)	Usually intentional, least concerning

The hierarchy is customizable via treemacs-git-directory-face-severity-list.

Custom function — a user-provided function that receives the git-info hash table for post-processing. The Python script resolves directory faces using severity logic as a baseline; the custom function then runs on the Emacs side to adjust the result (it operates on already-resolved severity faces, not raw status codes).

Files changed

• src/elisp/treemacs-customization.el — new defcustom variables
• src/elisp/treemacs-async.el — helper function, pass mode to Python, custom function post-processing
• src/scripts/treemacs-git-status.py — two-pass directory resolution for severity mode
• src/extra/treemacs-magit.el — pass mode arg, custom function post-processing

Backward compatibility

Default mode is modified, producing identical behavior to current code. No existing configuration breaks.

Test plan

• modified mode: verify directories show treemacs-git-modified-face (unchanged behavior)
• severity mode: git add new files only → parent directories show treemacs-git-added-face; add a modified file → directory switches to treemacs-git-modified-face
• Custom function mode: provide a lambda that remaps faces, verify it takes effect
• Magit integration: commit via magit, verify refresh uses correct mode