Integrate plugin MCP servers into apply

[yacosta738]

Goal

Extract MCP server declarations from supported plugins and merge them into AgentSync's existing MCP configuration pipeline.

Scope

• Read supported plugin MCP declarations, including standard .mcp.json layouts.
• Normalize server definitions into the existing McpServerConfig model.
• Merge plugin servers with explicit [mcp_servers.*] entries using a documented precedence rule.
• Generate the existing per-agent outputs, including Claude .mcp.json and Codex .codex/config.toml.
• Respect [mcp] enablement, merge/overwrite strategy, and --agents filtering.
• Include plugin origin in diagnostics and machine-readable output.
• Handle unsupported app-bound/authenticated MCP declarations explicitly.

Security requirements

• Never copy credentials from plugin content into committed configuration.
• Do not execute server commands during apply.
• Validate command, URL, headers, environment, and transport fields with existing safety rules.
• Preserve unrelated user-managed MCP servers according to current merge semantics.

Dependencies

Blocked by #574 for the source/lock contract. Coordinate with #575 for plugin staging paths.

Acceptance criteria

• A fixture plugin's MCP servers appear in the expected Claude and Codex outputs after agentsync apply.
• Explicit repository MCP configuration wins or loses according to a tested, documented rule.
• Dry-run reports changes without writing files.
• Existing MCP configs and unrelated servers remain intact under merge mode.
• Invalid or unsupported plugin MCP data fails with actionable diagnostics.
• Unit, integration, and output-contract tests cover supported agents.

[linear-code]

DALLAY-588