Add ReinOpenWeights adapter, closing GLAS-WP-0001-T05

glas_harness/reins/rein_openweights.py implements the Rein ABC by
shelling out to `rein-openweights run --task-file ... --no-hub
[--model ...]`, mirroring reins/rein_aharness.py exactly. Factored the
duplicated git_head/write_task_file logic into reins/_shared.py, used
by both adapters now. registry/reins/rein-openweights.yaml flipped to
status: implemented. 8 new tests (mocked subprocess), 18/18 passing.

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

src/glas_harness/reins/rein_openweights.py

@@ -0,0 +1,73 @@
+"""ReinOpenWeights — glas-harness adapter around the rein-openweights CLI.
+
+Same shape as reins/rein_aharness.py: rein-openweights's `run` command
+performs an entire bounded agentic session (credential acquisition,
+OpenRouter tool-calling loop, commit verification) as one opaque unit —
+no per-tool-call hooks exposed yet. dispatch_tool collapses the whole
+run into a single call.
+"""
+
+from __future__ import annotations
+
+import shutil
+import subprocess
+from typing import Any
+
+from glas_harness.contract import Rein, SandboxHandle, ToolCall, ToolResult
+from glas_harness.reins._shared import git_head, write_task_file
+
+
+class ReinOpenWeightsNotInstalled(RuntimeError):
+    pass
+
+
+class ReinOpenWeights(Rein):
+    def __init__(self, cli_bin: str = "rein-openweights", model: str | None = None) -> None:
+        self.cli_bin = cli_bin
+        self.model = model
+
+    def _bin(self) -> str:
+        resolved = shutil.which(self.cli_bin)
+        if not resolved:
+            raise ReinOpenWeightsNotInstalled(
+                f"'{self.cli_bin}' not found on PATH — install rein-openweights first"
+            )
+        return resolved
+
+    def start_session(
+        self, profile: dict[str, Any], inputs: dict[str, str], sandbox: SandboxHandle
+    ) -> dict[str, str]:
+        target_repo = (
+            inputs.get("target_repo")
+            or sandbox.reachability.get("workspace_dir")
+            or sandbox.reachability.get("remote_dir")
+        )
+        if not target_repo:
+            raise ValueError("no target_repo resolvable from inputs or sandbox reachability")
+
+        task_file = inputs.get("task_file") or write_task_file(
+            inputs["title"], inputs["description"], target_repo
+        )
+
+        return {
+            "sandbox_id": sandbox.sandbox_id,
+            "task_file": task_file,
+            "target_repo": target_repo,
+            "head_before": git_head(target_repo),
+        }
+
+    def dispatch_tool(self, session: dict[str, str], tool_call: ToolCall) -> ToolResult:
+        argv = [self._bin(), "run", "--task-file", session["task_file"], "--no-hub"]
+        if self.model:
+            argv += ["--model", self.model]
+        proc = subprocess.run(argv, capture_output=True, text=True)
+        ok = proc.returncode == 0
+        return ToolResult(ok=ok, output=proc.stdout, error=None if ok else proc.stderr)
+
+    def end_session(self, session: dict[str, str]) -> dict[str, str]:
+        head_after = git_head(session["target_repo"])
+        committed = bool(head_after) and head_after != session.get("head_before")
+        return {
+            "commit_sha": head_after or "",
+            "committed": str(committed),
+        }