M6: prompt mode + status + docs + release

[rwrife]

Closes #6

Ships the M6 milestone: prompt integration, a polished status command, real docs, and a release pipeline.

What changed

commit-sprout status — new subcommand. Compact, plain-text (script-friendly) summary: stage + health, current streak (with remembered best), last commit in whole days, and a days-until-wilt nudge.

commit-sprout prompt (+ --prompt flag) — new one-line glyph for shell prompt / tmux / starship. Built for the hot path:

• Read-only by default — a prompt renders on every command, so it skips the state write unless you pass --save.
• Fails silently outside a git repo — prints nothing, exits 0, so it can never break your shell.

Shared pipeline refactor — root/status/prompt now ride one runPipeline() (read git activity → load state → compute) + persist() helper, so the three commands stay consistent and DRY. No behavior change to the existing root render.

Docs — README now has: prebuilt-binary install from Releases, status/prompt usage with sample output, and copy-paste starship, tmux, bash, and zsh prompt snippets. Status banner bumped to v0.1; LICENSE link fixed.

LICENSE — MIT added.

Release — .goreleaser.yaml (CGO-free static binaries for linux/macos/windows × amd64/arm64, archives + checksums, version injected into --version from the tag) and .github/workflows/release.yml (runs GoReleaser on v* tags). Validated locally with goreleaser check and a snapshot build (version ldflag confirmed).

Checklist mapping

• --prompt emits a compact glyph + stage
• Prompt mode is fast + never blocks the shell (read-only default, silent-fail)
• status shows streak, last commit, days-until-wilt
• README: install, starship snippet, tmux snippet
• LICENSE (MIT)
• GoReleaser builds static binaries on tag

Testing

• gofmt -l . clean, go vet ./..., go build ./..., go test ./... all pass
• New cmd/status_test.go covers the status/streak/wilt phrasing helpers
• Manually verified status, prompt, --prompt, and silent-outside-repo behavior
• goreleaser check + snapshot build pass; --version ldflag injection confirmed

Not included

• Screenshots/asciinema (needs a real terminal capture) and the actual v0.1.0 tag cut — the release pipeline is ready; tagging is a maintainer action.