Formalize C0/C1/C2 change classification (#259)

## Summary
- **C0 (Quick Fix):** Now explicitly allows direct-to-main commits with no PR required — for low-risk, fix-forward-safe changes
- **C1 (Human Review):** New docs-first workflow with branch deployment (ArgoCD `--revision`, Ansible from checkout). Includes upgrade criteria for escalation to C2
- **C2 (Mikado Chain):** Introduces the **Mikado Branch Invariant** — strict commit ordering where card-introducing commits come first, followed by code progress, followed by card closures. Branch resets required when new prerequisites are discovered

Updates CLAUDE.md rules (3, 4, 8, 9) to reflect that C0 bypasses branching/PR requirements. Also updates ai-assistance-guide, how-to index, and docs-mikado task description.

## Files changed
- `CLAUDE.md` — rules and classification table
- `docs/how-to/agent-change-process.md` — full process rewrite
- `docs/tutorials/ai-assistance-guide.md` — branching and pitfalls sections
- `docs/how-to/how-to.md` — index description
- `mise-tasks/docs-mikado` — task description
- `docs/changelog.d/formalize-change-classification.doc.md` — changelog fragment

Reviewed-on: https://forge.ops.eblu.me/eblume/blumeops/pulls/259

docs/how-to/how-to.md

@@ -35,7 +35,7 @@ Task-oriented instructions for common BlumeOps operations. These guides assume y
 |-------|-------------|
 | [[review-documentation]] | Periodically review and maintain documentation |
 | [[review-services]] | Periodically review services for version freshness |
-| [[agent-change-process]] | C0/C1/C2 change classification and Mikado method for agents |
+| [[agent-change-process]] | C0/C1/C2 change classification and Mikado Branch Invariant |
 
 ## Operations