Derive OAuth authorization server strictly from gh-host in HTTP mode

[Copilot]

Summary

This change removes custom OAuth authorization-server overrides and keeps HTTP OAuth metadata bound to the GitHub host resolver. For GHES, setting --gh-host/GITHUB_HOST now remains the single configuration step for correct OAuth authorization server discovery.

Why

Allowing a separate authorization-server override introduced an unnecessary second source of truth and could diverge from host-derived behavior.

What changed

• Config surface simplification
  • Removed --authorization-server flag and corresponding Viper binding/env plumbing.
• HTTP server wiring
  • Removed AuthorizationServer from HTTP server config path so OAuth metadata uses host-derived resolution only.
• Docs alignment
  • Removed documentation references to custom authorization-server overrides.

# Supported configuration path
github-mcp-server http --gh-host https://ghe.example.com

MCP impact

• No tool or API changes
• Tool schema or behavior changed
• New tool added

Prompts tested (tool changes only)

• N/A (no tool changes)

Security / limits

• No security or limits impact
• Auth / permissions considered
• Data exposure, filtering, or token/size limits considered

Tool renaming

• I am renaming tools as part of this PR (e.g. a part of a consolidation effort)
  • I have added the new tool aliases in deprecated_tool_aliases.go
• I am not renaming tools as part of this PR

Note: if you're renaming tools, you must add the tool aliases. For more information on how to do so, please refer to the official docs.

Lint & tests

• Linted locally with ./script/lint
• Tested locally with ./script/test

Docs

• Not needed
• Updated (README / docs / examples)

[pachecocordovamoiseseduardo-byte]

👍