chore: Update SDLC instructions and contract

CLAUDE.md

@@ -174,9 +174,19 @@ Issues are the source of truth. See `.ai/workflow-contract.json` for complete wo
 - Every PR must link to at least one issue
 - Use Gitea MCP tools for issue/label/branch/PR operations
 - Labels: `status/backlog` -> `status/ready` -> `status/in-progress` -> `status/review` -> `status/done`
-- Branches: `issue-{index}-{slug}` (e.g., `issue-42-add-fuel-report`)
+- Branches: `issue-{parent_index}-{slug}` (e.g., `issue-42-add-fuel-report`)
 - Commits: `{type}: {summary} (refs #{index})` (e.g., `feat: add fuel report (refs #42)`)
 
+### Sub-Issue Decomposition
+Multi-file features (3+ files) must be broken into sub-issues for smaller AI context windows:
+- **Sub-issue title**: `{type}: {summary} (#{parent_index})` -- parent index in title
+- **Sub-issue body**: First line `Relates to #{parent_index}`
+- **ONE branch** per parent issue only. Never branch per sub-issue.
+- **ONE PR** per parent issue. Body lists `Fixes #N` for parent and every sub-issue.
+- **Commits** reference the specific sub-issue: `feat: add dashboard (refs #107)`
+- **Status labels** tracked on parent only. Sub-issues stay `status/backlog`.
+- **Plan milestones** map 1:1 to sub-issues.
+
 ## Architecture Context for AI
 
 ### 9-Container Architecture