Standardize future commit messages

[marcus]

Summary

• split built-in task summaries from richer agent-facing instructions and route prompts through that path
• add a repo-level commit message guide, installable commit template, and commit-msg hook for future commit consistency
• add tests for prompt fallback behavior and the new commit-msg validation hook

Testing

• go test ./internal/tasks ./internal/orchestrator ./cmd/nightshift/commands
• go test ./...