Git worktree support: reconstitute .git for container mounts

[jewzaam]

Problem

When act is run from a git worktree, container-based jobs fail because the .git entry is a file (pointer to the main repo's .git/worktrees/<name>/ directory) rather than a self-contained .git/ directory. Docker containers cannot follow this pointer back to the host filesystem, causing failures in actions/checkout and any step that needs git metadata.

Additionally, when a non-worktree repo contains worktree checkout directories (e.g., .claude/worktrees/ created by Claude Code), docker cp fails because the .git pointer files inside those subdirectories collide during tar extraction with "file exists" errors.

Relates to #657 (closed without resolution).

Proposed Fix

Two changes:

1. Worktree reconstitution (pkg/common/git/worktree.go)

When act detects that .git is a file (worktree indicator), it:

1. Parses the .git file to find the worktree's gitdir
2. Reads commondir to locate the main repo's .git/ directory
3. Creates a temp directory with a copy of the working tree (excluding .git)
4. Builds a self-contained .git/ directory by copying commondir contents and overlaying worktree-specific files (HEAD, index, etc.)
5. Uses this temp directory as the bind mount source for the container

The temp directory is cleaned up when the job container closes.

2. Skip .git entries in subdirectories (pkg/filecollector/file_collector.go)

The FileCollector used by CopyDir (which creates the tar archive for docker cp) now skips .git entries (files or directories) at all directory levels, not just the root. These are git-internal metadata and should never be copied into the container. This fixes the case where a parent repo contains worktree checkout directories — their .git pointer files no longer cause docker cp to fail with "file exists" errors.

Testing

• Unit tests for worktree detection, .git file parsing, commondir resolution, and the copy logic
• Manual testing with Docker/Podman against repos using git worktree

Environment

• Tested on Windows (Git Bash + Podman) and designed to work cross-platform
• Affects both --bind and volume-based (default) workspace modes

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This issue was created with assistance from Claude Code (Claude Opus 4.6).