[plugins] Phase 4: REST API + thv plugin CLI + content preview (THV-0077)

[JAORMX]

Goal: Expose plugins over REST and the thv plugin CLI, including content preview.

Deliverables:

• pkg/api/v1/plugins.go + plugins_types.go — REST under /api/v1beta/plugins (template skills.go), incl. /content + /marketplace.
• pkg/plugins/client/ — HTTP client (template pkg/skills/client/).
• cmd/thv/app/plugin*.go — cobra commands mirroring cmd/thv/app/skill*.go; add plugin marketplace generate (no skills analog).
• GetContent wiring (template skillsvc/content.go).

Dependency/gate: depends on Phase 3 PluginService interface frozen — can overlap late P3.

Testing/exit gate: each CLI command vs. mock registry; API handler tests (template skills_test.go); info shows hooks + "declared, NOT managed by ToolHive" for MCP.

Part of #5525
Refs RFC PR stacklok/toolhive-rfcs#77