feat(cmd): add current command to detect the active worktree

[fstr]

Add treehouse current, which reports whether the working directory is inside a managed worktree. It exits 0 when it is and non-zero otherwise, so it composes in scripts, prompts, and hooks.

Detection is authoritative rather than advisory: it derives the owning repository with git. FindMainRepoRootFrom (so pool resolution is correct from inside a linked worktree) and cross-checks the cwd against pool state via the new pool.Current, instead of trusting the $TREEHOUSE_DIR variable that only the get subshell exports. pool.Current guards on IsPoolDir so it never creates a pool directory as a side effect.

Output modes:

• default: worktree name, live status, and path (mirrors a status row)
• --path: only the absolute worktree path (silent when outside)
• --json: {"in_worktree":true,...} or {"in_worktree":false}

Non-zero "not here" exits use a new errSilent sentinel so Execute skips extra stderr decoration.