Harness foundation: INTENT, ADR-001, architecture, prototype adoption

- INTENT.md: three-layer model (blueprint/instance/harness), single
  shared runtime, never-become boundaries
- ADR-001 (accepted): DEC-2026-002 resolution — one harness repo for
  all projects; instances are declarative state in consuming repos
- docs/architecture.md: components, contracts (manifest, tool
  profiles, completion events, credential lanes), deployment shape
- agent_harness/: executor-worker prototype adopted and renamed
  (6/6 tests green); HARNESS-WP-0001 initial workplan (7 tasks)

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

examples/task-hello-sandbox.json

@@ -0,0 +1,9 @@
+{
+  "title": "Sandbox smoke: NOTES.md",
+  "description": "Create a file NOTES.md in the repository root containing a two-sentence description of what this sandbox repository is for (read README.md first). Commit it with message 'executor smoke: add NOTES.md'.",
+  "target_repo": "~/executor-sandbox",
+  "agent": "coach",
+  "labels": ["executor", "smoke"],
+  "completion_event_type": "executor_run",
+  "timeout_seconds": 600
+}