Cletus plugin kit: rules for coding agents, specification, host API, style guide, worker SDK and cletus-plugin tool

sdk/cmd/cletus-plugin/template/docs/plan.md.tmpl

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+# [[.Title]]: plan
+
+## What it does
+
+Counts the lines in a text file the user chooses, and saves a short summary
+next to it.
+
+## Chat tools
+
+- `[[.Short]]_count_lines`: reads a file and returns its line count. Read-only.
+- `[[.Short]]_save_summary`: writes `<file>.summary.txt` next to the file.
+  Mutating; checks `host.files.can_write` first.
+
+## Permissions
+
+- `files`: to read the file the user chose and write the summary beside it.
+
+## Dependencies
+
+- `[[.SDKModule]]`: the Cletus plugin SDK.
+
+## Notes for review
+
+- Direct file access: `internal/tools` reads and writes only paths returned
+  by `host.files.resolve`, after `host.files.can_write` for writes.