docs(coding-agent): completed full omp docs sync

Added a contributor-facing native crate map (docs/native-crates.md) covering pi-natives, pi-shell, pi-ast, pi-iso, pi-walker, pi_uu_grep, pi-uutils-ctx, and vendored brush crates, and linked it from natives-architecture.md and user-facing-packages.md.

Added a docs-index tool coverage test asserting every BUILTIN_TOOL_NAMES entry and injected custom tool (generate_image, tts) has a docs/tools/<name>.md page served by omp://.

Inlined tiny fail/buildPayloadText/checkDocsIndexFreshness helpers in generate-docs-index.ts per the project rule against single-expression named functions.

Fixes #3934

packages/coding-agent/test/internal-urls/docs-tool-coverage.test.ts

@@ -0,0 +1,37 @@
+import { describe, expect, it } from "bun:test";
+import * as fs from "node:fs";
+import * as path from "node:path";
+import { BUILTIN_TOOL_NAMES } from "@oh-my-pi/pi-coding-agent/tools/builtin-names";
+
+// Every shipped built-in tool that is exposed to the model in normal sessions
+// must have a docs/tools/<name>.md root doc served by `omp://`. File names use
+// underscores or hyphens; the test accepts either form so renaming the on-disk
+// page does not require coordinating with the wire name.
+const docsToolsDir = path.resolve(import.meta.dir, "../../../../docs/tools");
+
+const expectedDocPaths = (name: string): string[] => [
+	path.join(docsToolsDir, `${name}.md`),
+	path.join(docsToolsDir, `${name.replace(/_/g, "-")}.md`),
+];
+
+// Custom tools injected by the SDK (`packages/coding-agent/src/sdk.ts`) when
+// their settings are enabled. Built-in tool factories live in BUILTIN_TOOLS but
+// these custom tools are not present there, so the coverage list is explicit.
+const CUSTOM_TOOL_NAMES = ["generate_image", "tts"] as const;
+
+describe("omp:// root docs coverage", () => {
+	it.each([...BUILTIN_TOOL_NAMES])("documents builtin tool %s", name => {
+		const candidates = expectedDocPaths(name);
+		const present = candidates.find(candidate => fs.existsSync(candidate));
+		expect(
+			present,
+			`Missing docs/tools/<name>.md for built-in tool "${name}". Tried: ${candidates.join(", ")}.`,
+		).toBeDefined();
+	});
+
+	it.each([...CUSTOM_TOOL_NAMES])("documents injected custom tool %s", name => {
+		const candidates = expectedDocPaths(name);
+		const present = candidates.find(candidate => fs.existsSync(candidate));
+		expect(present, `Missing docs/tools/<name>.md for injected custom tool "${name}".`).toBeDefined();
+	});
+});