v0.7.1: notify-cmd input for the GitHub Action

[Codevena]

Summary

Exposes the v0.7.0 --notify-cmd hook to workflow users — patch release v0.7.1.

• New action input notify-cmd, one command per line (newline-separated on purpose: shell commands may legitimately contain commas, unlike the comma-separated label input; a YAML block scalar makes multi-line natural). Each non-empty trimmed line becomes a repeated --notify-cmd flag, passed injection-safe via env:.
• The action-smoke workflow now passes notify-cmd: "echo smoke-notify", exercising the wiring on every PR (dry-run never fires notifications by design).
• README action-inputs row, CHANGELOG, v0.7.1 bumps, fresh SHA256SUMS.

Test Plan

• Parsing loop verified standalone (trim, blank-line skip, commas preserved)
• YAML validation, bash -n per file, shellcheck clean, 20/20 integration tests
• Review gate: Codex PASS + Claude PASS (verified herestring trailing-newline, CRLF, injection path, checksums)
• CI green incl. action-smoke with the new input

Release steps after merge

1. git tag v0.7.1 && git push origin v0.7.1
2. git tag -f v1 v0.7.1 && git push origin v1 --force
3. gh release create v0.7.1 + install smoke test

🤖 Generated with Claude Code