feat: trimkit-symlink command for worktree setup

[josephfung]

Idea

Ship a trimkit-symlink CLI command that replaces the inline shell loop currently used to symlink gitignored files from a main checkout into a worktree. Instead of Claude generating shell constructs with variable assignments, semicolons, and for loops, the command would be:

trimkit-symlink /path/to/main /path/to/worktree .env .env.local data/

Goal

Eliminate the entire class of problems around the current symlink pattern:

• Permission bypass risk: The current inline loop uses semicolons (or newlines) which can chain arbitrary commands after an auto-approved prefix. A dedicated command means the permission rule is just Bash(trimkit-symlink:*) — flat, unambiguous, no shell constructs.
• Ordering gotcha: Today MAIN= must come first in the command so it matches Bash(MAIN=:*). A named command removes this fragile dependency.
• Circular symlink risk: The CLAUDE.md has a CRITICAL warning about cd not persisting and relative paths creating circular symlinks. The command can enforce this internally.
• Documentation burden: The current CLAUDE.md devotes multiple paragraphs and caveats to explaining the symlink pattern. This shrinks to one line.

Built-in safety checks

The command should:

• Refuse to create a circular symlink (source and destination resolve to the same path)
• Validate both directories exist before starting
• Refuse to overwrite non-symlink files (protect against clobbering real files in the worktree)
• Warn (not fail silently) when a requested item doesn't exist in the source directory

Acceptance criteria

• trimkit-symlink <main-dir> <worktree-dir> <item>... creates symlinks for each item
• Exits non-zero with a clear message if source or destination directory doesn't exist
• Exits non-zero if a symlink would be circular (source == destination after resolution)
• Skips (with warning to stderr) items that don't exist in the source
• Refuses to overwrite a non-symlink file in the destination (exits non-zero with message)
• Overwrites an existing symlink in the destination (re-pointing is safe)
• Has tests covering all of the above
• install.sh places it in ~/.trimkit/bin/
• CLAUDE.md template updated to use the new command instead of the inline loop
• Permission rule Bash(trimkit-symlink:*) added to settings template