feat: add spinloop up to start a directory's engine - #167

Adds spinloop up: one word to start the engine a directory holds — the fleet a fleet.yaml names, or the local server a Spinloop describes.

Summary

• New top-level spinloop up (cmd/spinloop/up.go). In a directory with a fleet.yaml it runs the fleet start path — a bare up starts every node, up <node> the named ones; elsewhere it runs serve's own body, so resolution, output and errors are serve's.
• A fleet.yaml wins over a Spinloop in the same directory. Positionals only — an unknown flag is refused, not forwarded.
• Tab completion: the fleet's node names in a fleet directory, the Spinloop slot elsewhere, silent on failure.
• User docs: new docs/commands/up.md, the command-table row, and cross-references from the README and the serve, fleet, completion, alias, spinloop-file and getting-started pages.
• The up-command openspec change is implemented, archived, and synced to openspec/specs/up-command/spec.md.

Implementation details

• The branches reuse runFleetDrive/fleetStartCall and runServe rather than re-implementing either, so up and the commands it wraps cannot disagree. A bare fleet start refuses to run without a target, which is why a bare up is the --all form.
• The completion slot is the only CWD-dependent one: node names where ./fleet.yaml parses, the Spinloop slot elsewhere, nothing where the fleet file is present but unreadable — __complete never errors.