fix: defer MCP command loading to background

[OscSer]

What does this PR do?

Prevents Command.state() from waiting on MCP prompt loading, allowing user commands to appear immediately on startup without being blocked by MCP initialization.

Key changes:

• Separates user command loading (synchronous) from MCP command loading (background)
• Initializes MCP commands asynchronously via Command.initMCPCommands() called during bootstrap
• Publishes command.updated event when MCP commands are ready, syncing the TUI without blocking
• Reduces startup impact from slow or failing remote MCPs (which attempt two transports sequentially: StreamableHTTP → SSE)
• Adds guard clause in Session.prompt.command() to handle missing commands gracefully

How did you verify your code works?

• Tested with multiple remote MCPs configured
• Verified custom user commands appear immediately on startup without waiting for MCP initialization
• Confirmed MCP commands appear progressively as each MCP finishes loading

Issues

Fixes #8868
Fixes #8017