Add update-documentation skill

.claude/skills/update-documentation/SKILL.md

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+---
+name: update-documentation
+description: Use when the user asks to update project documentation, or after significant code changes that may have made docs/architecture.md or docs/project-conventions.md stale
+---
+
+# Update Documentation
+
+Updates `docs/architecture.md` and `docs/project-conventions.md` to reflect recent codebase changes.
+
+## Workflow
+
+For each documentation file (`docs/architecture.md`, `docs/project-conventions.md`):
+
+### 1. Find the last documentation update
+
+```bash
+git log -1 --format="%H %as %s" -- docs/architecture.md
+git log -1 --format="%H %as %s" -- docs/project-conventions.md
+```
+
+### 2. List all commits since then
+
+```bash
+git log --oneline <last-commit-hash>..HEAD
+```
+
+If there are no commits after the last update, the file is already up to date — skip it.
+
+### 3. Analyze each commit for documentation impact
+
+For each commit, check if it introduced changes relevant to the documentation file:
+
+- **architecture.md**: new/removed/renamed modules, schemas, contexts, workers, routes, external integrations, supervision tree changes, database changes, PubSub topics, LiveView/LiveComponent additions
+- **project-conventions.md**: new patterns established across 3+ commits, new conventions visible in code review, changed testing patterns, new error handling approaches, new UI/template conventions
+
+Use `git show --stat <hash>` and `git show <hash>` to understand each commit. Focus on structural changes, not bug fixes or minor tweaks.
+
+### 4. Prepare the update
+
+- Read the current documentation file
+- Draft the specific edits needed (additions, modifications, removals)
+- Present the proposed changes to the user for review before applying them
+
+### 5. Get human approval
+
+Show the user:
+- Which commits drove the changes
+- What sections are being added, modified, or removed
+- The exact content of each edit
+
+Only apply changes after explicit approval.
+
+### 6. Apply and verify
+
+- Apply the approved edits
+- Commit with a message describing what was updated (e.g., "Update architecture docs" or "Update project conventions")