[feature] sp promote — graduate a scratch into the current repo

[rwrife]

Pitch: A throwaway sometimes turns out to matter. sp promote <id> graduates a scratch into the current repo — moves the file in and opens it — so the good ones escape the reaper.

Acceptance criteria

• sp promote <id> [dest] moves the scratch out of the store into the current working dir (or dest)
• Removes the scratch from the index once promoted (it's the repo's responsibility now)
• Refuses to overwrite an existing file without --force
• Opens the promoted file in $EDITOR after moving (unless --no-open)
• Clear confirmation message showing the new path

Notes

• This is the natural "oops, keep this" escape hatch. Pairs well with the future secret-tripwire check.

[rwrife]

Implemented in #20 — sp promote <id> [dest] moves a scratch out of the store into the working tree and drops it from the index, with --force overwrite guard and --no-open. All five acceptance criteria covered, with store- and CLI-level tests. Leaving the merge to the next triage cycle once CI is green.