docs: add agent guidance files and templates

.agent/templates/task-brief.md

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+# Task Brief
+
+## Task ID
+
+`YYYY-MM-DD-short-name`
+
+## Goal
+
+Describe the intended change in one or two sentences.
+
+## Context
+
+Add links, errors, user stories, examples, existing behavior, or relevant files.
+
+## Non-goals
+
+- Do not ...
+- Do not ...
+
+## Acceptance criteria
+
+- [ ] ...
+- [ ] ...
+- [ ] ...
+
+## Verification
+
+Run the relevant commands for this project:
+
+- `...`
+- `...`
+
+## Risk level
+
+Choose one: Fast / Standard / Cautious
+
+## Notes for Claude
+
+Any constraints, preferred approach, migration concerns, compatibility requirements, or rollout notes.