Add sync-files command

[sjquant]

Why

Private, gitignored files like .env.local and .npmrc often need to follow every new worktree, but configuring an afterCreate hook for file copying is manual and puts a common workflow in the wrong abstraction.

Changes

• Add gji sync-files list|add|remove to manage per-repo syncFiles entries in global config.
• Preserve existing tilde-keyed per-repo config entries, support parent-position --json, emit JSON errors in JSON mode, and avoid config churn for no-op removes.
• Reuse sync file path validation before writing config, and clean up empty per-repo entries when the last file is removed.
• Wire the command into CLI registration, shell completions, README docs, and website docs.
• Cover the command with integration tests, including end-to-end copying into a new worktree and regressions from review.

Verification

• pnpm build
• pnpm test
• pnpm --dir website build
• pnpm --dir website typecheck