feat: add commit-msg hook and simplify session-commit

[Cragsmann]

Summary

• Add Husky pre-commit (lint) and commit-msg hooks
• commit-msg hook validates: conventional commit type prefix, authorship trailer (Co-Authored-By or Signed-off-by), no em dashes
• Merge, fixup, and squash commits are exempt
• Simplify session-commit: remove progress log and blocker migration steps, add hook failure handling
• Remove redundant commit-user command

Issue: SRVOCF-950

Test plan

• Verify tandem commit (with Co-Authored-By) passes the hook
• Verify autonomous commit (with Signed-off-by) passes the hook
• Verify commit without authorship trailer is rejected
• Verify commit without conventional type prefix is rejected
• Verify commit with em dash is rejected
• Verify merge commits bypass the hook
• Run /session-commit and confirm it produces a valid commit