sp new --stdin: headless scratch capture from a pipe (#27)

[rwrife]

Closes #27

What changed

Adds a headless capture path to sp new so scratchpatch can ingest content programmatically instead of always shelling out to $EDITOR:

• --stdin — read content from standard input (pipes, heredocs)
• --content "<text>" — one-liner content
• --from-file <path> — seed from an existing file

Any of these suppresses $EDITOR. Exactly one source per invocation (conflicts error out). Interactive sp new is unchanged when none are set.

Implementation

• New store.WriteContent seeds a scratch's content file and refreshes recorded size (reuses the existing create/index/live-path machinery).
• captureContent helper gathers + validates the source; TTY-with-no-pipe on --stdin refuses rather than hanging; --content "" is a deliberate empty scratch.
• Because content lands on disk, sp ls's live secret scan flags piped-in credentials with 🔑 and sp promote guards them — same as editor scratches. No metadata/source-of-truth changes.

Acceptance criteria

• --stdin reads stdin, no $EDITOR
• Honors --ttl/--ext/--tag
• --content and --from-file companions
• Empty stdin/--content handled deliberately (empty scratch)
• Stable created scratch <id> anchor preserved
• Secret scanner runs on captured content (🔑 in sp ls)
• --stdin on a bare TTY doesn't hang (refuses)
• Tests: stdin capture, flag combos, secret detection, editor-not-invoked, conflicting sources
• README documents the pipe workflow
• go build/go vet/go test ./.../gofmt clean

Testing

go build ./..., go vet ./..., gofmt -l (clean), go test ./... all green; manual smoke test of pipe capture, --content, and the 🔑 marker on a piped secret.