docs(readme): document the wt sync command

[epodivilov]

wt sync is registered in src/index.ts but had no README section — the only mention was the hooks.post-sync row in the config options table.

Adds a ### wt sync section under Commands, placed after wt update and before wt doctor, in the same shape as the neighbouring sections: description, flag table, commented examples block, then prose.

Documented surface, checked against src/cli/commands/sync.ts and src/application/use-cases/sync-worktrees.ts:

• positional branch — syncs all worktrees when omitted, errors when no worktree matches
• --dry-run — reports the plan, writes nothing, skips post-sync hooks
• --force — overwrites existing files at copy destinations

The section also covers when to reach for the command (after editing copy/symlinks in config, to push those into worktrees that already exist) and links to Options for the post-sync hook. One Features bullet added at the top.

Examples were run against the local build (pnpm dev -- sync --help, --dry-run, positional branch, unknown branch). pnpm lint and pnpm typecheck both exit 0.

Edits are confined to the Features list and the Commands section, staying clear of the hooks/configuration area being changed on fix/hooks-windows-warning.