feat: Multi-org GitHub App support

[wpfleger96]

Summary

Enables the GitHub MCP server to work with repositories across multiple GitHub organizations by supporting per-org GitHub App installation tokens.

Key changes:

• Parse GITHUB_INSTALLATION_ID_<ORG> environment variables for per-org installation IDs
• New MultiOrgClientFactory that creates authenticated clients per organization
• Updated GetClientFn signature to accept owner parameter
• All ~40 tool handlers updated to pass owner to client factory
• Backwards compatible: falls back to GITHUB_INSTALLATION_ID if no org-specific ID configured

Environment Variables

Variable	Purpose
GITHUB_INSTALLATION_ID_SQUAREUP	squareup org installation ID
GITHUB_INSTALLATION_ID_AFTERPAYTOUCH	AfterpayTouch org installation ID
GITHUB_INSTALLATION_ID	Fallback (backwards compatibility)

Test plan

• Build: go build ./cmd/...
• Tests: go test ./...
• Manual test with multi-org env vars set

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