Formalize the CLI as the first Channel extension (GLAS-WP-0003)

channels/base.py: GatewayInvocation dataclass + Channel ABC
(parse_invocation/render_result), mirroring Rein one layer up -- a
channel turns an external invocation into what run_task_through_rein
needs, and turns its result back into whatever the invocation medium
expects.

channels/cli_channel.py: CLIChannel, a pure refactor of what cli.py did
inline -- argparse Namespace -> GatewayInvocation, result dict ->
json.dumps(..., indent=2). cli.py's run handler is now a thin
construct-parse-invoke-render wrapper.

docs/channel-contract.md documents the pattern and sketches (without
building) a second channel.

Found and fixed a real bug while live-testing this refactor:
ReinAharness's default cli_bin was still "agent-harness", stale from
before HARNESS-WP-0002-T02's rename.

4 new tests (27/27 passing), plus a real live run through
python3 -m glas_harness.cli against rein-aharness/real Claude Code
confirming identical output shape and a real commit.

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

src/glas_harness/cli.py

@@ -1,9 +1,8 @@
-"""glas-harness CLI — minimal gateway invocation (GLAS-WP-0001-T04)."""
+"""glas-harness CLI — thin wrapper around CLIChannel (GLAS-WP-0003)."""
 
 from __future__ import annotations
 
 import argparse
-import json
 import sys
 
 
@@ -23,18 +22,21 @@ def main(argv: list[str] | None = None) -> int:
     args = parser.parse_args(argv)
 
     if args.command == "run":
+        from glas_harness.channels.cli_channel import CLIChannel
         from glas_harness.gateway import run_task_through_rein
 
+        channel = CLIChannel()
+        invocation = channel.parse_invocation(args)
         result = run_task_through_rein(
-            sandbox_profile=args.sandbox_profile,
-            repo=args.repo,
-            title=args.title,
-            description=args.description,
-            actor=args.actor,
-            project=args.project,
-            report_to_hub=not args.no_hub,
+            sandbox_profile=invocation.sandbox_profile,
+            repo=invocation.repo,
+            title=invocation.title,
+            description=invocation.description,
+            actor=invocation.actor,
+            project=invocation.project,
+            report_to_hub=invocation.report_to_hub,
         )
-        print(json.dumps(result, indent=2))
+        print(channel.render_result(result))
         return 0 if result["tool_ok"] else 1
 
     return 1