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>

agent_harness/taskspec.py

@@ -0,0 +1,48 @@
+"""Task specification the worker consumes.
+
+MVP source is a JSON file matching what the activity-core issue-core sink
+emits per rule action (task_template/description/target_repo/labels), plus
+executor-specific fields. A future source polls issue-core directly.
+"""
+
+from __future__ import annotations
+
+import json
+from dataclasses import dataclass, field
+from pathlib import Path
+
+
+class TaskSpecError(ValueError):
+    pass
+
+
+@dataclass
+class TaskSpec:
+    title: str
+    description: str
+    target_repo: Path
+    agent: str = "coach"
+    labels: list[str] = field(default_factory=list)
+    hub_task_id: str | None = None
+    completion_event_type: str = "executor_run"
+    timeout_seconds: int = 900
+
+    @classmethod
+    def from_file(cls, path: str | Path) -> "TaskSpec":
+        raw = json.loads(Path(path).read_text())
+        missing = {"title", "description", "target_repo"} - set(raw)
+        if missing:
+            raise TaskSpecError(f"task spec missing field(s): {', '.join(sorted(missing))}")
+        target = Path(raw["target_repo"]).expanduser()
+        if not (target / ".git").is_dir():
+            raise TaskSpecError(f"target_repo is not a git repository: {target}")
+        return cls(
+            title=str(raw["title"]),
+            description=str(raw["description"]),
+            target_repo=target,
+            agent=str(raw.get("agent", "coach")),
+            labels=[str(label) for label in raw.get("labels", [])],
+            hub_task_id=raw.get("hub_task_id"),
+            completion_event_type=str(raw.get("completion_event_type", "executor_run")),
+            timeout_seconds=int(raw.get("timeout_seconds", 900)),
+        )