fix: resolve symlinks in git-common-dir for worktree support

[newbeelee048]

Problem

When using git worktree on macOS (where /tmp is a symlink to /private/tmp), the git-common-dir path returned by git differs between the main repo and worktrees:

• Main repo: returns .git → resolved to /tmp/worktree-test/main/.git
• Feature worktree: returns /private/tmp/worktree-test/main/.git (absolute path)

This causes different project tags to be generated, preventing memory sharing between worktrees of the same repository.

Solution

Use realpathSync to resolve symlinks in the getGitCommonDir function, ensuring consistent paths regardless of whether git returns a relative or absolute path, or whether the path contains symlinks.

Testing

# Before fix
Main repo tag:      opencode_project_34e2192f1926e08d
Feature worktree:   opencode_project_834e8685c25e514b
# Different tags → memories not shared ❌

# After fix
Main repo tag:      opencode_project_834e8685c25e514b
Feature worktree:   opencode_project_834e8685c25e514b
# Same tags → memories shared ✅

Changes

• Added realpathSync and existsSync imports from node:fs
• Modified getGitCommonDir to resolve symlinks before returning the path

Fixes #51