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/CHANGELOG.md

@@ -2,6 +2,14 @@
 
 ## [Unreleased]
 
+### Added
+
+- Added `resolveSchemaRef(ref, root)`, which returns the subschema a local `$ref` (`#`, `#/$defs/Item`) names in a schema, with URI-decoded and JSON Pointer unescaped segments ([#11803](https://github.com/earendil-works/pi/issues/11803))
+
+### Fixed
+
+- Fixed `schemaToType()`, `renderDeclarations()`, `renderToolSignature()`, and `renderToolSample()` throwing `URIError` for a local `$ref` with a malformed percent escape (`#/$defs/%E0`), so one such tool schema failed every caller that renders all tools. The reference now renders as `unknown`, like a reference that names nothing ([#11803](https://github.com/earendil-works/pi/issues/11803))
+
 ## [1.5.0] - 2026-11-02
 
 ### Changed