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/.gitignore

@@ -2,3 +2,4 @@ lib/
 src/client/bsk-ui.nomodule.css
 .bsk-ui.input.css
 .bsk-ui.raw.css
+src/skill-content.generated.ts