fix(coding-agent): follow local schema refs in tool search and codemode summaries (#11850)

Fixes #11803.

**Problem**

`tool_search` builds a tool's search text from its parameter schema but
never followed `$ref`. MCP servers (Pydantic, FastMCP) often put nested
models in `$defs`:

```json
{ "properties": { "options": { "$ref": "#/$defs/options" } },
  "$defs": { "options": { "properties": { "mode": { "description": "reconciliation" } } } } }
```

Search text was `Inspect records options`, so
`tool_search("reconciliation")` found nothing. Durable's copy of
`tool_search` had the same gap. The codemode `resolves to` summary had
the same blind spot for output schemas: a field behind a `$ref` showed
as `structuredContent?` with no field names.

**Fix**

- pi-codemode exports its existing local JSON Pointer resolver as
`resolveSchemaRef(ref, root)`. A malformed percent escape now resolves
to nothing instead of throwing `URIError` (this also affected
`schemaToType()`).
- coding-agent and Durable `schemaText()` follow local `$ref`s. Each
referenced subschema is read once, so recursive and shared definitions
neither loop nor inflate BM25 term counts. Unreferenced `$defs` and
remote refs stay out.
- coding-agent `describeFields()`/`describeUnionFields()` resolve `$ref`
chains against the output schema root.

**Tests**

- `packages/codemode/test/declarations.test.ts`: escaped, remote,
missing, malformed refs.
- `packages/coding-agent/test/tool-search.test.ts`: refs, escaped
segments, recursion and shared definitions, unused definitions.
- `packages/coding-agent/test/suite/agent-session-codemode.test.ts`:
summary through root, field, const, and union refs.
- `packages/durable/test/tool-search.test.ts`: end-to-end load via a
ref'd description; unused definition does not match.

`npm run check` passes.

packages/codemode/test/declarations.test.ts

@@ -1,5 +1,11 @@
 import { describe, expect, it } from "vitest";
-import { renderDeclarations, renderToolSample, renderToolSignature, schemaToType } from "../src/index.ts";
+import {
+	renderDeclarations,
+	renderToolSample,
+	renderToolSignature,
+	resolveSchemaRef,
+	schemaToType,
+} from "../src/index.ts";
 
 const execute = () => undefined;
 
@@ -222,6 +228,25 @@ describe("schemaToType", () => {
 	});
 });
 
+describe("resolveSchemaRef", () => {
+	const item = { type: "string" };
+	const root = { $defs: { item, "a/b": item, "c~d": item, "e f": item } };
+
+	it("resolves local references with escaped segments", () => {
+		expect(resolveSchemaRef("#", root)).toBe(root);
+		expect(resolveSchemaRef("#/$defs/item", root)).toBe(item);
+		expect(resolveSchemaRef("#/$defs/a~1b", root)).toBe(item);
+		expect(resolveSchemaRef("#/$defs/c~0d", root)).toBe(item);
+		expect(resolveSchemaRef("#/$defs/e%20f", root)).toBe(item);
+	});
+
+	it("returns undefined for remote, missing, and malformed references", () => {
+		expect(resolveSchemaRef("other.json#/$defs/item", root)).toBeUndefined();
+		expect(resolveSchemaRef("#/$defs/missing", root)).toBeUndefined();
+		expect(resolveSchemaRef("#/$defs/%E0%A4%A", root)).toBeUndefined();
+	});
+});
+
 describe("tool declarations", () => {
 	it("renders signatures with normalized identifiers", () => {
 		expect(