docs: add agent guidance files and templates

.agent/templates/plan.md

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+# Plan
+
+## Task classification
+
+Fast / Standard / Cautious
+
+## Assumptions
+
+- ...
+
+## Files inspected
+
+- `path/to/file`
+
+## Files likely to change
+
+- `path/to/file`
+
+## Files that should not change
+
+- `path/to/file`
+
+## Implementation steps
+
+1. ...
+2. ...
+3. ...
+
+## Tests to add or update
+
+- ...
+
+## Verification commands
+
+- `...`
+
+## Risks and ambiguities
+
+- ...
+
+## Stopping conditions
+
+Stop and ask before continuing if:
+
+- scope needs to expand;
+- existing behavior contradicts the brief;
+- verification cannot be run;
+- a required dependency or secret is missing.