feat(builtins): add patch command

[chaliy]

Summary

patch applies unified diff patches to files. Combined with the existing diff builtin, this enables full diff-based code modification workflows — particularly valuable for AI agents that generate and apply code changes.

Requested Flags

Flag	Description
-p NUM	Strip NUM leading path components from filenames
-R	Reverse — unapply the patch
-i file	Read patch from file instead of stdin
--dry-run	Check if patch applies without modifying files
-N	Ignore patches that appear already applied
-b	Create .orig backup files
--fuzz=NUM	Allow NUM lines of fuzz for context matching

Use Cases

# Apply a unified diff
diff -u old.py new.py > fix.patch
patch -p0 < fix.patch

# Apply patch with path stripping (common for git-format patches)
patch -p1 < feature.patch

# Dry run to check if patch applies cleanly
patch --dry-run -p1 < update.patch

# Reverse a patch
patch -R -p1 < applied.patch

# Agent workflow: generate diff, review, apply
diff -u original.rs modified.rs > changes.patch
cat changes.patch  # review
patch -p0 < changes.patch

Implementation Notes

• Must parse unified diff format (---/+++/@@ headers, +/-/ lines)
• Should work on VFS files
• Context matching with fuzz is important for robustness
• Reject patches for files outside VFS sandbox
• Exit code: 0 = success, 1 = some hunks failed, 2 = serious trouble

[chaliy]

Resolved in PR #585 or #588