Smriti was repo-local wiring: every command needed an explicit <space>, and
the SessionStart hook hard-coded the space name, so working across projects
and sessions meant re-stating the space constantly.
Add a `.smriti.json` attachment file at the repo root binding the repo to a
space. The CLI resolves `<space>` from it — walking up from the working
directory the way git finds `.git` — so `<space>` is now optional on the
everyday commands (state, current, metrics, claim, checkpoint, branch,
worktree). `smriti attach <space>` is the explicit binding verb (`smriti
attach` with no argument shows the current binding); `smriti init` also
writes the attachment. The generated SessionStart hook is now space-agnostic
— `smriti state --compact` resolves from the attachment, so the hook is
identical for every project and survives re-attaching.
`space delete` still requires an explicit space. No backend changes —
attach/init reuse the existing set_project_root endpoint.
