Implement the harness contract and prove it against rein-aharness (GLAS-WP-0001-T02/T03/T04)

- src/glas_harness/contract.py: Rein ABC (start_session/dispatch_tool/
  end_session), SandboxHandle/ToolCall/ToolResult, per docs/harness-contract.md.
- reins/rein_aharness.py: adapter around the rein-aharness CLI. Collapses
  a whole `agent-harness run` into one dispatch_tool call for now (no
  per-tool hooks yet — tracked in rein-aharness HARNESS-WP-0002-T03);
  verifies success via new-commit detection, mirroring rein-aharness's
  own success signal.
- gateway.py + cli.py: resolves a sand-boxer sandbox, runs one task
  through a rein, tears the sandbox down.
- registry/reins/*.yaml (rein-aharness implemented, rein-openweights
  planned) and profiles/harness.agent-dev{,-local}.yaml, pairing with
  sand-boxer's profile.agent-dev and the new profile.bwrap-local.

Tested against a real local git repo + mocked SandboxManager/CLI
subprocess (10 tests, all passing). A real live run against the actual
Claude Code CLI is deliberately left for a human-triggered follow-up —
not executed autonomously since it spends real API credits/credentials.

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

profiles/harness.agent-dev.yaml

@@ -0,0 +1,10 @@
+id: harness.agent-dev
+version: "1.0.0"
+rein: rein-aharness
+sandbox_profile: profile.agent-dev
+tool_profile: green-commit-only
+metadata:
+  latency_class: standard
+  # Pairs with sand-boxer's profile.agent-dev (ext.compose-ssh, remote
+  # host over SSH). Use harness.agent-dev-local for fast local iteration
+  # without the SSH hop.