fix(server): scope gh's child git processes to the selected account

[enisze]

Problem

Per-project GitHub account pinning (#7) covers git push/fetch and the integrated terminal, but not the git processes gh itself spawns. GitHubCli.execute passed gitHubAccountGhEnv — token only, no credential config — so:

• gh pr create pushing a branch with no upstream, and
• gh pr checkout fetching

still fell back to the machine credential helper and acted as whichever account it had cached.

Fix

• GitHubCli.execute now passes gitHubAccountAuthEnv, so gh's child git processes inherit the same credential config as our own git calls.
• gitHubAccountAuthEnv appends its GIT_CONFIG_* pairs after any GIT_CONFIG_COUNT already in the parent env. Previously it hardcoded COUNT=2 and claimed indices 0/1, silently discarding an inherited runtime git config.
• Silent fallback is now logged. A failed or empty gh auth token warns instead of quietly acting as the ambient account — that silence is what made the original bug hard to diagnose.
• Tests for the path, which shipped without any: env shape (github.com + enterprise), inherited-config offset, malformed GIT_CONFIG_COUNT, gh child-process env, and terminal spawn env.
• Drive-by: GitHubAccountResolver.test.ts was missing worktreeBranchPrefix after fix(server): restore access to existing worktrees #7 changed the contract, which broke tsgo --noEmit on main. Fixed.
• Docs: the note in source-control-providers.md still said plain git pushes were unscoped. Replaced with what actually holds now, plus the two real limits (terminals pick up the account at spawn; SSH remotes can't be pinned).

Verification

Against two real gh accounts on one machine:

globally active: enisze
pinning to:     ezsns
git would authenticate as: ezsns   ✅

without the pin (pre-#7 behaviour):
username=15271433                  ← stale OS keychain entry, neither account

tsgo --noEmit clean, 259 tests pass across sourceControl/vcs/terminal, lint clean. Targeted checks only, per AGENTS.md — CI owns the full suite. Not exercised through the UI.

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