feat(dsh-plugin-browserskill): publish the BSK skill through ctx.skills

The plugin exposed only tools; the BrowserSkill agent skill never reached
the harness's <available_skills> catalog. Register it as an embedded runtime
skill (progressive disclosure: catalog entry resident, body on skill-tool
invocation) via the official ctx.skills seam — silent no-op when the
composition lacks it.

Content is build-time assembled so there is one source of truth:
skill/prelude.md (committed, dsh-specific: tool<->CLI map, owned-session
semantics, plugin-only overrides) + the canonical repo-root skill/SKILL.md
verbatim (the same file crates/bsk-cli/build.rs mirrors; frontmatter
stripped, name/description supplied by the registration). Generated into
src/skill-content.generated.ts (gitignored) — registration and every
pre-step snapshot are pure in-memory reads: no disk, no process, no daemon.

Scripts: build:skill chained into build/test/typecheck (same pattern as
build:css); biome excludes the generated artifacts. Tests: +3 (catalog
content, in-memory weight, silent degradation). 116/116 green.

packages/dsh-plugin-browserskill/src/index.ts

@@ -17,6 +17,7 @@ import { registerObservationRoutes } from "./observation-http";
 import { KeyedExecutor } from "./queue";
 import { type BskRunner, createBskRunner } from "./runner";
 import { SessionRegistry } from "./sessions";
+import { registerBskSkill } from "./skill";
 import { type PluginConfig, registerTools } from "./tools";
 
 export const name = "dsh-plugin-browserskill";
@@ -80,6 +81,10 @@ export function apply(
   });
 
   registerTools({ ctx, runner, registry, config: resolved, observation, queue });
+  // Progressive disclosure of the BSK agent skill (catalog entry resident,
+  // body on demand) through the official skill seam; silent no-op when the
+  // composition lacks it.
+  const unregisterSkill = registerBskSkill(ctx);
   // Route registration rides ctx.inject: the webServer service may be provided
   // AFTER this plugin loads, and in headless compositions it never appears (the
   // callback simply never runs, leaving the rest of the plugin unaffected).
@@ -110,6 +115,7 @@ export function apply(
   // are swallowed so one stale handle cannot abort the rest.
   ctx.effect(() => {
     return () => {
+      unregisterSkill();
       removeRoutes();
       observation.dispose();
       runner.killAll();

packages/dsh-plugin-browserskill/src/skill.ts

@@ -0,0 +1,45 @@
+/**
+ * Progressive disclosure of the BrowserSkill agent skill through the harness's
+ * official skill seam (`ctx.skills`). The catalog entry (name + description)
+ * is resident; the body is loaded only when the model invokes the `skill`
+ * tool. The markdown is build-time assembled into a static module (dsh
+ * prelude + the canonical CLI skill body verbatim), so registration and every
+ * pre-step catalog snapshot are pure in-memory reads — no disk, no process,
+ * no daemon.
+ */
+
+import type { Context } from "@deepseek-ai/cordis";
+import {
+  BSK_SKILL_DESCRIPTION,
+  BSK_SKILL_MARKDOWN,
+  BSK_SKILL_NAME,
+} from "./skill-content.generated";
+
+/** Structural view of the skill registry seam (absent in bare compositions). */
+interface SkillsLike {
+  register(skill: {
+    name: string;
+    description: string;
+    content: string;
+    source?: string;
+  }): () => void;
+}
+
+/**
+ * Publish the BSK skill as an embedded runtime skill. Returns the unregistration
+ * disposer. A composition without the `skills` service (or with dsh-tool-skill
+ * retired) leaves the rest of the plugin unaffected — the no-op is silent.
+ */
+export function registerBskSkill(ctx: Context): () => void {
+  const skills = ctx.get("skills") as SkillsLike | undefined | null;
+  if (skills == null || typeof skills.register !== "function") {
+    return () => {};
+  }
+  return skills.register({
+    name: BSK_SKILL_NAME,
+    description: BSK_SKILL_DESCRIPTION,
+    content: BSK_SKILL_MARKDOWN,
+    // Prompt-visible origin bucket: packaged with a plugin, not user/project files.
+    source: "bundled",
+  });
+}