[feature] sp new --stdin / quick-capture from a pipe (headless scratch creation)

[rwrife]

Pitch

Today sp new always shells out to $EDITOR — perfect for interactive use, useless in a pipeline or from another tool/AI agent. There's no way to say "take this text and stash it as a scratch" without a human opening an editor. Add a headless capture path so scratchpatch can ingest content programmatically:

pytest -q 2>&1 | sp new failing-tests --stdin --tag ci
sp new note --stdin <<'EOF'
remember to revoke that token
EOF
curl -s https://api.example.com/thing | sp new resp --ext json --stdin

This turns scratchpatch into a sink for throwaway output (logs, API responses, generated snippets), which is squarely on-mission ("give every throwaway a home with an expiry") and directly serves the AI-agent-hygiene angle from PLAN.md — agents can park temp output in the reaped store instead of littering the repo.

Why it fits

• Reuses the exact create/index path sp new already uses; only difference is content comes from stdin (or --content/--from-file) instead of an editor round-trip.
• Composes with the existing secret tripwire: piped-in content should still get scanned so a secret gets flagged (🔑) at capture time.
• Pure additive flag on an existing command; interactive sp new behavior unchanged when --stdin is absent.

Acceptance criteria

• sp new [name] --stdin reads content from standard input and creates the scratch without launching $EDITOR.
• Honors existing --ttl, --ext, --tag flags on the same invocation.
• Sensible companions: --content "<text>" for a one-liner and/or --from-file <path> to seed from an existing file (pick at least --stdin; the others are nice-to-have if cheap).
• Empty stdin is handled deliberately (either refuse with a clear message or create an empty scratch — document the choice).
• On success, prints the same tombstone-flavored confirmation as interactive sp new (still leads with the stable created scratch <id> anchor so scripts/tests keep working).
• Newly captured content is run through the secret scanner so sp ls shows the 🔑 marker and sp promote guards it, same as editor-created scratches.
• --stdin + a TTY with no piped input doesn't hang forever (guard/allow but document).
• Tests cover: stdin capture, flag combos, secret detection on piped content, and that $EDITOR is not invoked. go build/go vet/go test ./.../gofmt clean.
• README documents the headless/pipe workflow with examples.

[rwrife]

Opened #29 with the headless capture path (--stdin / --content / --from-file, editor-suppressed, secret-scanned, TTY-hang-guarded). Leaving merge to the next triage cycle once CI is green.