feat: add local coding agent harness

docs/coding-agent-task-template.md

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+# Coding Agent Task Template
+
+Copy this block into an issue or coding-agent request. One task should have one
+primary outcome; split unrelated outcomes instead of hiding them in acceptance
+criteria.
+
+```text
+Task kind: bug | feature | refactor | docs | policy | release
+Goal (one primary outcome):
+Current behavior and evidence:
+Acceptance criteria:
+In scope (packages/files/surfaces):
+Out of scope:
+Compatibility constraints:
+Interface impact (commands/flags/output/errors/exit codes/Schema):
+Safety or data-mutation constraints:
+Expected validation:
+Known environment limitations:
+```
+
+For a command or remote-interface task, add the smallest concrete invocation
+and contract evidence available:
+
+```text
+CLI path and example argv:
+Current --help or Schema excerpt:
+Remote method and request/response shape, if relevant:
+Expected stdout/stderr and exit behavior:
+Mutation preview/confirmation behavior:
+```
+
+Do not paste credentials, tokens, private endpoints, or production business
+data. Use redacted fixtures and say which evidence is unavailable. Save the
+filled block and run `make coding-agent-task TASK=path/to/task.md` to validate
+it before implementation.