chore: standardize future commit messages

[marcus]

Summary

• split built-in task summaries from agent-facing instructions and give commit-normalize actionable prompt guidance
• add a forward-looking commit message standard with template, hook validation, and docs
• make make install-hooks work in linked worktrees by resolving absolute hook paths and writing commit.template to worktree-local config

Testing

• go test ./internal/tasks ./internal/orchestrator ./cmd/nightshift/commands
• go test ./...
• make install-hooks
• make -C -f /Users/marcus/code/nightshift/Makefile install-hooks