feat: add commit message normalizer (library + commit-normalize subcommand)

[iamlasse]

Summary

Adds a Conventional Commits commit-message normalizer, exposed both as a reusable Go library (internal/normalizer) and as a nightshift commit-normalize subcommand.

Normalization rules

• Lowercase the subject line.
• Strip trailing periods from the subject.
• Trim leading/trailing whitespace (overall and per line).
• Separate subject and body with exactly one blank line (inserted if missing).
• Enforce the type(scope)!?: subject shape — lowercases the type/scope, normalizes the colon-space and ! breaking marker.
• Wrap body paragraphs to <=100 columns (greedy word wrap); footer lines (Token: value / Token #123) are left verbatim.
• Warn when the subject exceeds 72 characters or lacks a type prefix.

CLI

nightshift commit-normalize reads a message from a positional file path, --file, or stdin, prints the normalized message to stdout, emits any warnings to stderr, and exits non-zero on empty/invalid input.

$ echo 'Fix: Add OAuth.' | nightshift commit-normalize
fix: add oauth

Tests

Table-driven tests in internal/normalizer/normalizer_test.go cover: clean input, lowercase + period stripping, scope/breaking-marker normalization, missing type prefix, oversized subject, trailing-whitespace trimming, blank-line insertion, subject/body/footer structure, and empty-input errors. Additional tests assert body wrapping stays <=100 columns and that footers are not wrapped. go build ./..., go vet, and go test ./... all pass.

Assumptions / notes

• The plan assumed the repo was empty and would be bootstrapped from scratch. In practice marcus/nightshift already contains a Go module, so this integrates into the existing module (github.com/marcus/nightshift) and follows the established cobra command pattern rather than running go mod init or rewriting main.go.
• "Lowercase the subject" is implemented literally: the entire subject line (type, scope, and description) is lowercased. If the project prefers to lowercase only the type(scope) prefix and preserve the description's casing, that is a one-line change.
• The 72-char subject limit and 100-col body wrap are exposed as SubjectLimit / WrapLimit constants.

Nightshift-Task: commit-normalize
Nightshift-Ref: https://github.com/marcus/nightshift