github: resolve SSH host aliases when classifying remotes

[maruthang]

Fixes #283245

Problem

When a user's ~/.ssh/config defines a custom Host alias for github.com (e.g. Host gh-personal -> HostName github.com), their git remotes look like git@gh-personal:owner/repo.git. The GitHub extension's getRepositoryFromUrl only matched the literal github.com, so repositoryHasGitHubRemote() returned false and every GitHub feature (Open on GitHub, status-bar Copilot indexed badge, share providers, branch protection, hover commands, etc.) silently disabled for these users.

Fix

1. New extensions/github/src/sshConfig.ts
   • parseSshConfig(text) - literal-alias parser; skips */?/! patterns.
   • SshConfigHostResolver - lazily reads ~/.ssh/config plus /etc/ssh/ssh_config on POSIX, caches the parsed map, exposes resolveSync(alias).
   • Module-level sharedSshConfigHostResolver singleton.
2. util.ts - getRepositoryFromUrl(url, sshResolver = sharedSshConfigHostResolver) keeps its synchronous signature and gains a generic git@<host>:<owner>/<repo> fallback that returns { owner, repo } only when <host> resolves to literal github.com via the resolver. The default-injected resolver keeps the function unit-testable.
3. extension.ts::activate - calls sharedSshConfigHostResolver.load() once on activation so the synchronous lookup has data.

Scope

• Only aliases mapping to literal github.com are recognized - GitHub Enterprise hosts are intentionally not auto-recognized.
• Missing or unreadable ~/.ssh/config fails silently, preserving current behavior for users without one.
• No new dependencies. Only the GitHub extension is touched.

Tests

extensions/github/src/test/sshConfig.test.ts covers:

• parseSshConfig: simple blocks, multi-block, multi-alias Host lines, wildcard skip, case-insensitive = separators, comments, duplicate-alias precedence.
• getRepositoryFromUrl with an injected stub resolver: literal github.com, alias -> github.com, alias -> non-github host, unknown alias.

[vs-code-engineering]

📬 CODENOTIFY

The following users are being notified based on files changed in this PR:

@lszomoru

Matched files:

• extensions/github/src/extension.ts
• extensions/github/src/sshConfig.ts
• extensions/github/src/test/sshConfig.test.ts
• extensions/github/src/util.ts