Make delivery "placement" (install vs export) data-driven in the target manifest

Background

Follow-up from #28 (review comment). #28 externalized the target architecture registry so the choice grid is now fully data-driven (TARGETS.map(...)). This issue tracks the remaining follow-up and is not a regression in #28.

Problem

There's a second axis — placement (install-in-place vs export/download) — that's still inferred from architecture identity in the renderer rather than declared in the manifest. This is the same "one target, many edits" smell that #20 was about, just on the delivery dimension:

• src/Library.tsx:496 — default useState<SkillArchitecture>("scout")
• src/Library.tsx:966 — architecture === "cowork" ? "export" : "install"
• src/Library.tsx:1143-1161 — "Added to Scout" / "Add to Scout" copy + the Scout-only Download button, gated on architecture === "scout"

Adding a new target (e.g. "Copilot Studio (skill, export-only)") today still requires touching these renderer branches.

Proposed approach

Lift placement into the existing target manifest as capabilities rather than identities:

{ id: "scout",  installTargetLabel: "Scout",
  targets: [
    { kind: "skill",      placements: ["install", "export"] },
    { kind: "automation", placements: ["install"] },
  ] }
{ id: "cowork", installTargetLabel: "Microsoft 365 Copilot",
  targets: [
    { kind: "skill", placements: ["export"] },
  ] }

Then the renderer derives:

• default = first enabled target
• placement buttons = target.placements
• copy = installTargetLabel

…so no architecture string literals remain in src/. Adding "Copilot Studio (skill, export-only)" becomes one manifest entry + one catalogue file, with no renderer edits.

Scope / boundary (where to stop)

Keep the same UI-safe boundary already enforced for catalogue text:

• Config = data/capabilities — labels, enabled, kinds, placements, icon key.
• Behavior stays in code — skill vs automation genuinely branch (different builder, plan schema, IPC, render target), so those remain typed primitives; the manifest just decides whether each is offered.
• Don't let config turn into a UI DSL. For any genuinely bespoke per-architecture UX later, a component registry keyed by SkillArchitecture beats piling more fields into config.

References

• PR #28 (Externalize target architecture registry)
• #20 (original "add a target, edit five separate lists" problem)
• Original proposal by @adilei in https://github.com/microsoft/skill-recorder/pull/28#issuecomment-5136440249