T-126: C clide shell client + _argv unwrap in the dispatcher

Third slice of T-99. After this `clide status` actually does
something when typed in a shell.

* native/clide-cli/clide.c — ~250 LOC C. Walks CWD up to .git,
  hashes the workspace root with FNV-1a 64-bit (byte-for-byte
  identical to the Dart side, pinned via reference vectors in
  paths_test.dart), opens the per-workspace socket, and ships argv
  across the wire as `{cmd:"_argv", args:{argv:[...]}}`.
* lib/src/cli/argv_dispatch.dart — registers the `_argv` sentinel
  command on the dispatcher. The handler runs the T-125 parser on
  the embedded argv and either re-dispatches the unwrapped request
  through the same dispatcher or returns the pre-built error
  response. Keeps the parser in Dart so the C side stays dumb.
* lib/src/ipc/paths.dart — fnv1a64Hex hoisted to a public helper +
  fixed to format as unsigned (Dart `int` is signed int64; the high
  bit lit a leading minus that broke the cross-language compare).
  Reference-vector tests added against the FNV reference.
* `make clide-cli` builds it via the host `cc`; output lands at
  native/<platform>/clide and is gitignored. Test
  test/cli/clide_cli_e2e_test.dart compiles + exercises the full
  round-trip; skips cleanly when no cc is on PATH.
* CONTRIBUTING.md gets a "C clide shell client" section.

T-128 (delete legacy IPC) unblocked.

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

lib/main.dart

@@ -38,6 +38,7 @@ import 'package:clide/src/daemon/pane_commands.dart';
 import 'package:clide/src/daemon/pql_commands.dart';
 import 'package:clide/src/editor/registry.dart' show EditorRegistry;
 import 'package:clide/src/git/client.dart';
+import 'package:clide/src/cli/argv_dispatch.dart';
 import 'package:clide/src/ipc/envelope.dart';
 import 'package:clide/src/ipc/server.dart';
 import 'package:clide/src/panes/event_sink.dart';
@@ -115,6 +116,7 @@ Future<void> main() async {
     registerGitCommands(dispatcher, gitClient, eventSink);
     final pql = PqlClient(workDir: workRoot, toolchain: tc);
     registerPqlCommands(dispatcher, pql);
+    registerArgvUnwrap(dispatcher);
     return dispatcher;
   }