fix(mcp): run checkForUpdates async for mcp and serve subcommands

[jgarrone82]

Problem

checkForUpdates runs synchronously in main() before cmdMCP is called (cmd/engram/main.go:534). This means the MCP server does not start listening until the GitHub API call completes or times out.

// main.go:534 — runs BEFORE cmdMCP
if result := checkForUpdates(version); ... {
    fmt.Fprintln(os.Stderr, result.Message)
}
cmdMCP(cfg) // ← never reached until the HTTP call above finishes

Root cause

• Uses http.DefaultClient with no transport-level timeout (only a 2s context.WithTimeout)
• On Windows, DNS resolution and TCP operations can hang at the OS level beyond the context deadline — Go cancels the context but the underlying WSARecv call may not unblock immediately
• Claude Code has a stream idle timeout of idleDeadlineMs=300000 (5 minutes). If MCP never starts responding, Claude Code waits the full 5 minutes then kills the connection

Observed impact

• Measured MCP startup of 6417ms vs 1775ms baseline in debug logs (Windows 11)
• Suspected root cause of intermittent 5-minute hangs reported by users on Windows

Fix

Run the update check in a goroutine so cmdMCP starts immediately:

go func() {
    if result := checkForUpdates(version); result.Status != versioncheck.StatusUpToDate && result.Message != "" {
        fmt.Fprintln(os.Stderr, result.Message)
    }
}()
cmdMCP(cfg)

Interactive subcommands (save, search, tui, etc.) keep the synchronous check so the update message appears before command output.

Environment

• Windows 11, Claude Code with Engram MCP plugin
• Related: engram update with gentle-ai #263 (GitHub API 403 — same checkForUpdates call)

Closes #269

[Alan-TheGentleman]

Thanks for chasing #269. The direction is right, but I am closing this PR rather than merging the stale branch as-is because I am rebuilding the startup fix on current main with the rest of the issue batch. I will reference this PR in the replacement work so the original context is not lost. Closing the PR, not rejecting the underlying fix.