Rename system extension layer to base for clarity

Replace all references to the "system" layer with "base" to better reflect
its role as the foundational extension/tool layer in panto's hierarchy.
The layer hierarchy now consistently uses: project > user > base.

Includes:
- Update agent README and build.zig documentation
- Refactor tool registry and config module to support layered lookups
- Add TestHarness abstraction for cleaner test setup
- Improve JSON serialization with wire-encoded tool names
- Add glob pattern matching for tool/extension discovery

1 files changed, 8 insertions, 4 deletions

diff --git a/libpanto/src/root.zig b/libpanto/src/root.zig
index de62cfa..68f8e7d 100644
--- a/libpanto/src/root.zig
+++ b/libpanto/src/root.zig
@@ -19,11 +19,15 @@ pub const ToolDecl = tool_source.ToolDecl;
 pub const ToolCall = tool_source.Call;
 pub const ToolCallResult = tool_source.CallResult;
 pub const ToolRegistry = tool_registry.ToolRegistry;
+pub const Config = config.Config;
+pub const ProviderConfig = config.ProviderConfig;
 
-// Internal modules. Not part of the public API — callers construct providers
-// via `provider.Provider.init(allocator, io, config)`, which picks the right
-// concrete implementation based on the config's API style. These are exposed
-// here only so `refAllDecls` picks up their tests.
+// Internal modules. Not part of the public API — callers drive turns via
+// `provider.streamStep(allocator, io, &config, conv, receiver)` (or via the
+// `Agent`, which holds a swappable `*const Config`). The process-global HTTP
+// client is initialized with `config.initHttp` / torn down with
+// `config.deinitHttp`. These impls are exposed here only so `refAllDecls`
+// picks up their tests.
 const openai_chat_json = @import("openai_chat_json.zig");
 const provider_openai_chat = @import("provider_openai_chat.zig");
 const anthropic_messages_json = @import("anthropic_messages_json.zig");