M4: agent registry and bundled markdown agents

Adds harness-core::agent, an agent registry loading bundled markdown agent definitions (designer, explorer, fixer, librarian, oracle, orchestrator) with override support.

Cargo.lock

@@ -695,6 +695,7 @@ dependencies = [
  "schemars",
  "serde",
  "serde_json",
+ "serde_yaml_ng",
  "tempfile",
  "thiserror 2.0.18",
  "tokio",
@@ -803,6 +804,12 @@ dependencies = [
  "foldhash",
 ]
 
+[[package]]
+name = "hashbrown"
+version = "0.17.1"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "ed5909b6e89a2db4456e54cd5f673791d7eca6732202bbf2a9cc504fe2f9b84a"
+
 [[package]]
 name = "hashlink"
 version = "0.9.1"
@@ -1041,6 +1048,16 @@ dependencies = [
  "winapi-util",
 ]
 
+[[package]]
+name = "indexmap"
+version = "2.14.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "d466e9454f08e4a911e14806c24e16fba1b4c121d1ea474396f396069cf949d9"
+dependencies = [
+ "equivalent",
+ "hashbrown 0.17.1",
+]
+
 [[package]]
 name = "indoc"
 version = "2.0.7"
@@ -1827,6 +1844,19 @@ dependencies = [
  "serde",
 ]
 
+[[package]]
+name = "serde_yaml_ng"
+version = "0.10.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "7b4db627b98b36d4203a7b458cf3573730f2bb591b28871d916dfa9efabfd41f"
+dependencies = [
+ "indexmap",
+ "itoa",
+ "ryu",
+ "serde",
+ "unsafe-libyaml",
+]
+
 [[package]]
 name = "sharded-slab"
 version = "0.1.7"
@@ -2357,6 +2387,12 @@ version = "0.2.0"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "1fc81956842c57dac11422a97c3b8195a1ff727f06e85c84ed2e8aa277c9a0fd"
 
+[[package]]
+name = "unsafe-libyaml"
+version = "0.2.11"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "673aac59facbab8a9007c7f6108d11f63b603f7cabff99fabf650fea5c32b861"
+
 [[package]]
 name = "untrusted"
 version = "0.9.0"