Allow adopting provider MCP manifests

[Dkm0315]

Summary

• Add explicit provider MCP adoption via muster plugins reuse <provider> --adopt-mcp <id> and --adopt-all-mcps.
• Keep discovery read-only by default, while adoption writes only Muster-owned MCP config handles.
• Never copy provider secrets or OAuth tokens; adopted HTTP MCPs require Muster login/test after config.
• Thread the same adoption flags through chat with /plugins reuse <provider> --adopt-mcp <id>.
• Add regression coverage for HTTP/OAuth adoption, stdio adoption, missing MCP IDs, redaction, and cleanup.

Validation

• CI=true pnpm --filter @musterhq/cli typecheck
• CI=true pnpm exec tsx --test packages/cli/test/cli.test.ts
• git diff --check

Stack

• Base: Add chat-native explicit MCP setup workflows #70 chat-native explicit MCP setup
• This PR bridges provider-cache discovery and explicit Muster-owned MCP config.