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/coding-agent/test/suite/agent-session-codemode.test.ts

@@ -230,6 +230,50 @@ describe("AgentSession codemode tool", () => {
 		expect(description("count")).toBe("count tool\n\nCodemode: `tools.count(args)` resolves to `number`.");
 	});
 
+	it("summarizes output fields behind local schema references", async () => {
+		const outputTool = (name: string, outputSchema: Record<string, unknown>): AgentTool => ({
+			name,
+			label: name,
+			description: `${name} tool`,
+			parameters: Type.Object({}),
+			outputSchema: Type.Unsafe(outputSchema),
+			execute: async () => ({ content: [], details: {} }),
+		});
+		const report = outputTool("report", {
+			$ref: "#/$defs/Report",
+			$defs: {
+				Report: {
+					type: "object",
+					properties: { kind: { $ref: "#/$defs/Kind" }, summary: { $ref: "#/$defs/Summary" } },
+					required: ["kind", "summary"],
+				},
+				Kind: { const: "ledger" },
+				Summary: { type: "object", properties: { total: { type: "number" } }, required: ["total"] },
+			},
+		});
+		const result = outputTool("result", {
+			anyOf: [{ $ref: "#/$defs/Ok" }, { $ref: "#/$defs/Failed" }],
+			$defs: {
+				Ok: { type: "object", properties: { value: { type: "string" } }, required: ["value"] },
+				Failed: { type: "object", properties: { error: { type: "string" } }, required: ["error"] },
+			},
+		});
+		const harness = await setup([
+			(pi: ExtensionAPI) => {
+				for (const tool of [report, result]) pi.registerTool(createToolDefinitionFromAgentTool(tool));
+			},
+		]);
+		harness.session.setActiveToolsByName(["report", "result", "codemode"]);
+		const description = (name: string) =>
+			harness.session.agent.state.tools.find((tool) => tool.name === name)?.description ?? "";
+		expect(description("report")).toBe(
+			'report tool\n\nCodemode: `tools.report(args)` resolves to `{ kind: "ledger", summary: { total } }`.',
+		);
+		expect(description("result")).toBe(
+			"result tool\n\nCodemode: `tools.result(args)` resolves to `{ value } | { error }`.",
+		);
+	});
+
 	// #10343
 	it("shows the guidelines of tools that do not fit the inline budget through describeTool()", async () => {
 		const harness = await setup();