Experimental: Route GitHub API traffic through a custom host

Organizations can now route a GitHub host's API traffic through a gateway using the new per-host api_host configuration:

# Route API traffic for github.com through a gateway
gh config set api_host gh-gateway.example.com --host github.com

# Read the configured API host
gh config get api_host --host github.com

The original host remains in use for authentication, Git remotes, and browser URLs.

What's Changed

✨ Features

• Honor per-host api_host routing across GitHub API requests by @williammartin in #14104
• Expose api_host through gh config get and gh config set by @williammartin in #14332
• Add webhook as an official extension by @williammartin in #14326
• Print full command help after command misuse when gh is invoked by a coding agent by @niik in #14198

🐛 Fixes

• fix(api): disable telemetry for unauthenticated GHES requests using absolute hostnames by @williammartin in #14337

📚 Docs & Chores

• Fix discussion acceptance test flags by @williammartin in #14321
• Clarify supported GHES versions by @williammartin in #14324
• Improve automated issue triage analysis by @sergiou87 in #14318
• Record attachment counts in telemetry by @BagToad in #14327

Dependencies

• chore(deps): bump golang.org/x/crypto from 0.55.0 to 0.56.0 by @babakks in #14331
• chore(deps): bump Go toolchain from 1.26.7 to 1.26.8 by @babakks in #14330
• chore(deps): bump github.com/cli/go-gh/v2 from 2.15.0 to 2.16.0 by @williammartin in #14338

Full Changelog: v2.99.0...v2.100.0