perf: add inventory cache for stateless server patterns

[SamMorrowDrums]

Summary

Add CachedInventory to build tool/resource/prompt definitions once at startup rather than per-request. This is particularly useful for the remote server pattern where a new server instance is created per request.

Problem

In stateless deployments (like the remote MCP server), a new server and inventory is created for each incoming request. This means AllTools(t), AllResources(t), and AllPrompts(t) are called repeatedly, rebuilding all ~130 tool definitions including JSON schema generation on every request.

Related: modelcontextprotocol/go-sdk#685

Solution

Add a CachedInventory that:

• Builds all tools/resources/prompts once at startup
• Uses sync.Once for thread-safe initialization
• Returns pre-populated builders for per-request use

API

// Initialize once at startup (typically in main() or server init)
github.InitInventoryCache(translator)

// Then on each request, get a builder with pre-cached tools
inv := github.CachedInventoryBuilder().
    WithReadOnly(cfg.ReadOnly).
    WithToolsets(cfg.EnabledToolsets).
    WithFeatureChecker(checker).  // Still evaluated per-request!
    Build()

Key Features

• InitInventoryCache(t) - Called once at startup with your translator
• CachedInventoryBuilder() - Returns a builder pre-populated with cached definitions
• Per-request configuration still works - read-only, toolsets, feature flags, filters
• Thread-safe via sync.Once
• Backward compatible - NewInventory(t) still works without caching

Why This Works

The elegance is preserved because:

1. Tools are still self-describing - The GetMe(t), GetTeams(t) pattern stays the same
2. No code changes needed for tool definitions - They work as before
3. Feature flags work per-request - They're evaluated in AvailableTools(ctx), not at definition time
4. Translations resolved once - Since remote server uses NullTranslationHelper anyway, this is fine

For Remote Server Integration

// In server startup/init (once)
func init() {
    github.InitInventoryCache(translations.NullTranslationHelper)
}

// Per-request handler
func handleRequest(ctx context.Context, req Request) {
    inv := github.CachedInventoryBuilder().
        WithReadOnly(cfg.ReadOnly).
        WithToolsets(cfg.Toolsets).
        WithFeatureChecker(createFeatureChecker(ctx, req.User)).
        Build()
    
    // Feature flags are evaluated here, per-request
    tools := inv.AvailableTools(ctx)
    // ...
}

Testing

• Added comprehensive unit tests in inventory_cache_test.go
• All existing tests pass
• script/lint passes