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>

Makefile

@@ -268,6 +268,27 @@ dugite-fetch: ## Download and extract the dugite-native git distribution.
 dugite-clean: ## Remove the dugite-native directory.
 	rm -rf $(DUGITE_DIR)
 
+# -- clide-cli ----------------------------------------------------------
+# The C `clide` shell client that talks to the in-process IPC server
+# (T-99 / T-126). One source file, no third-party deps; the build
+# target picks up whatever `cc` is on PATH.
+
+CLIDE_CLI_SRC := native/clide-cli/clide.c
+CLIDE_CLI_BIN := native/$(if $(filter Darwin,$(shell uname -s)),macos,linux)-$(shell uname -m | sed 's/x86_64/x64/;s/aarch64/arm64/')/clide
+CC ?= cc
+
+.PHONY: clide-cli
+clide-cli: $(CLIDE_CLI_BIN) ## Compile the C `clide` shell client.
+
+$(CLIDE_CLI_BIN): $(CLIDE_CLI_SRC)
+	@mkdir -p $(dir $(CLIDE_CLI_BIN))
+	$(CC) -std=c99 -O2 -Wall -Wextra -o $(CLIDE_CLI_BIN) $(CLIDE_CLI_SRC)
+	@echo "==> built $(CLIDE_CLI_BIN)"
+
+.PHONY: clide-cli-clean
+clide-cli-clean: ## Remove the compiled C `clide` client.
+	rm -f $(CLIDE_CLI_BIN)
+
 # -- security -------------------------------------------------------------
 
 .PHONY: security