fix: support monorepo/multi-repo workspaces with nested git sub-projects

[can2049]

Summary

When opening a workspace that is not itself a git repository but contains nested git sub-projects (monorepo/multi-repo layout), fresh failed to:

• Recognize sub-projects as git repositories
• Display git status decorations in the file explorer
• Show correct branch name in the status bar
• Execute git blame, git grep, git find-file, and other git commands

This PR fixes all git-related functionality to work correctly in monorepo setups.

Root Causes

1. Symlink handling in readDir: The editor.readDir() backend used std::fs::DirEntry::file_type() which does not follow symlinks. In monorepos, .git is often a symlink to the actual git directory — it was incorrectly reported as is_dir=false, preventing plugins from detecting git repos.

2. Missing cwd in git command execution: Most plugins called editor.spawnProcess("git", args) without specifying a cwd parameter, causing git commands to execute in the workspace root (which is not a git repo in monorepo setups). This caused all git operations to fail with "not a git repository" errors.

3. Single-level sub-repo discovery: The file explorer and git index watcher only scanned immediate subdirectories. Multi-level structures like workspace/group/project/ were not discovered.

Changes

Core (Rust)

File	Change
quickjs_backend.rs	read_dir now follows symlinks via std::fs::metadata() to correctly report is_dir for .git symlinks
file_operations.rs	resolve_git_index uses BFS with max depth 3 to discover nested sub-repos

Plugins (TypeScript)

File	Change
git_explorer.ts	Added recursive discoverSubRepos(dir, maxDepth=3) for multi-level repo discovery
git_statusbar.ts	Added getGitCwd() helper — prioritizes active buffer's directory for branch detection
git_blame.ts	Passes editor.pathDirname(filePath) as cwd for git blame and git show
git_find_file.ts	Uses active buffer's directory as cwd for git ls-files
git_log.ts	Passes explicit cwd for git show
live_grep.ts	Git-grep availability check falls back to active buffer's directory
merge_conflict.ts	Passes file's directory as cwd for git show :0:path
code-tour.ts	Passes explicit cwd for git rev-parse
audit_mode.ts	All ~20 git spawn calls now use state.repoRoot || editor.getCwd() as cwd

Design Decisions

• Active buffer context: For per-file operations (blame, status bar, find-file), the file's own directory is used as cwd. Git automatically resolves the containing repository via rev-parse --show-toplevel.
• Recursive discovery with depth limit: Sub-repo scanning stops at depth 3 and prunes at .git boundaries (won't enter a git repo to find submodules — those are managed by git itself).
• Hidden directory filtering: Directories starting with . are skipped during discovery to avoid scanning .git, .cache, node_modules etc.
• Backward compatible: For single-repo workspaces, behavior is unchanged — the explicit cwd resolves to the same directory that was previously used implicitly.

Test Plan

• Project compiles without errors
• Open a monorepo workspace (top-level dir with multiple git sub-projects)
• Verify file explorer shows git status decorations for sub-projects
• Verify status bar shows correct branch when editing a file in a sub-project
• Verify git blame works for files in sub-projects
• Verify git find-file (ls-files) lists files from the active sub-project
• Verify git grep searches within the active sub-project
• Verify Review Diff mode works for sub-project changes
• Open a single-repo workspace and verify all features still work as before

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