Base pack: add commitlint pre-commit hook

[Fieldnote-Echo]

Source

Grippy review finding F-003 (LOW, confidence 80%). Validated by alpha.

Problem

CLAUDE.md prescribes conventional commits (feat:, fix:, chore:, etc.) but nothing enforces it. The pre-commit config has no commit-msg hook.

Fix

Add conventional-pre-commit hook to the base pack's .pre-commit-config.yaml.j2 template.

Found by Grippy (Devstral Q4 24b, local-first)