fix: Detect non-standard default branch names

[charlesvien]

Problem

Git setup fails with fatal: invalid reference main when a repo's default branch isn't main or master, because detection only checked those two names and callers silently fell back to hardcoded "main".

Changes

1. Add init.defaultBranch config and current HEAD as fallbacks in detectDefaultBranch()
2. Fix detectDefaultBranchWithFallback() to try HEAD before hardcoding "main"
3. Fix workspace service .catch(() => "main") to use current branch
4. Fix git service GitHub API fallback to return empty instead of guessing "main"
5. Add unit tests for all default branch detection strategies

How did you test this?

Manually + added unit tests

[charlesvien]

• fix: Remove snapshot APIs that deadlock libuv thread pool #1338
• fix: Detect non-standard default branch names #1336 👈 (View in Graphite)
• feat: Add prompt history to task input and fix command center text selection #1335
• fix: Only send 1M context beta header for supported models #1334
• main

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[charlesvien]

Merge activity

• Mar 30, 2:57 PM UTC: A user started a stack merge that includes this pull request via Graphite.
• Mar 30, 3:09 PM UTC: Graphite rebased this pull request as part of a merge.
• Mar 30, 3:18 PM UTC: @charlesvien merged this pull request with Graphite.