What does this PR do?

• Implement lazy loading for MCP tools to reduce initial context bloat by 95%+
• Add mcp_load_tools meta-tool for on-demand tool loading
• Add mcp_lazy_load experimental config flag (off by default)

Problem

When multiple MCP servers are enabled, all tool definitions load into the agent's context at session startup. A single MCP server like GitHub can add 15-20k+ tokens just for tool schemas. With several MCPs enabled, context bloat can exceed 50k tokens before the user even sends a message. This is even mentioned in the docs: https://opencode.ai/docs/mcp-servers/

Solution

Lazy loading pattern: Instead of loading full tool schemas at startup, we:

1. Show a lightweight index in the system prompt with server names and tool names only (~150 tokens vs ~1.5k)
2. Provide a mcp_load_tools meta-tool that loads specific tools on-demand
3. The agent loop naturally picks up newly loaded tools on the next iteration

Example Flow

System prompt shows:
"playwright (20 tools): browser_snapshot, browser_click, browser_navigate, ..."

User: "Take a screenshot of google.com"

Agent calls: mcp_load_tools("playwright", ["browser_navigate", "browser_snapshot"])
→ Tools loaded into session state

Agent calls: playwright_browser_navigate({ url: "https://google.com" })
Agent calls: playwright_browser_snapshot()

Changes
File: config/config.ts
Changes: Add mcp_lazy_load to experimental options
────────────────────────────────────────
File: session/index.ts
Changes: Add mcpLoadedTools to Session.Info schema
────────────────────────────────────────
File: mcp/index.ts
Changes: Add index(), loadToolsForSession(), modify tools() and create()
────────────────────────────────────────
File: session/system.ts
Changes: Add mcpIndex() for system prompt generation
────────────────────────────────────────
File: session/prompt.ts
Changes: Include MCP index in system prompt, pass session state to MCP.tools()
────────────────────────────────────────
File: tool/mcp-load-tools.ts
Changes: New meta-tool implementation
────────────────────────────────────────
File: tool/registry.ts
Changes: Register meta-tool conditionally

How did you verify your code works?

• Enable with "experimental": { "mcp_lazy_load": true } in config
• Verify MCP servers appear in system prompt with tool names
• Verify mcp_load_tools tool is available
• Ask agent to use MCP tools, confirm it calls mcp_load_tools first
• Verify loaded tools work correctly after loading
• Verify backward compatibility with flag disabled (default)

Closes #8277