[WIP] Fix runtime failure in push_to_pull_request_branch for multi-repo workflows

[Copilot]

• Fix build_checkout_manifest.cjs: add git ls-remote --symref origin HEAD fallback when git symbolic-ref --short refs/remotes/origin/HEAD fails, and also set origin/HEAD locally so the MCP server can use it
• Add 2 new tests for the new ls-remote fallback in build_checkout_manifest.test.cjs
• Validate existing tests pass (379 files pass, 1 pre-existing failure in mount_mcp_as_cli unrelated to this change)

• Fixes runtime: push_to_pull_request_branch resolves branch/base against workspace root in multi-repo checkout when subrepo origin/HEAD is unset #47638

[github-actions]

🤖 PR Triage

Field	Value
Category	bug
Risk	🔴 High
Priority Score	55/100
Score Breakdown	Impact: 30, Urgency: 15, Quality: 10
Action	defer — WIP draft; wait for agent to complete work

High-impact runtime bug (push_to_pull_request_branch multi-repo failure). Defer until the agent finalizes implementation.

Generated by 🔧 PR Triage Agent · sonnet46 · 26.8 AIC · ⌖ 4.99 AIC · ⊞ 5.7K · ◷

[github-actions]

Hey @Copilot 👋 — thanks for tackling this runtime issue in push_to_pull_request_branch! This fix is well-scoped and addresses the root cause identified in #47638 (missing origin/HEAD resolution in multi-repo checkouts).

The approach of adding a git ls-remote --symref origin HEAD fallback is solid, and your plan to add tests for the new fallback path looks good. The fix directly addresses the symptoms (workspace-root branch detection fallback) by ensuring the manifest has proper default_branch values for subrepos.

This is ready to proceed — looking forward to seeing the implementation land! 🚀

Generated by ✅ Contribution Check · sonnet46 · 64.4 AIC · ⌖ 10.5 AIC · ⊞ 6.2K · ◷

[github-actions]

🤖 PR Triage — §30214847023

Field	Value
Category	bug
Risk	🔴 High
Priority	52/100
Breakdown	Impact 30 · Urgency 17 · Quality 5
Action	defer
Status	DRAFT — WIP

Rationale: Runtime failure fix for push_to_pull_request_branch in multi-repo workflows — critical-path change but WIP/draft with no CI. Defer until undrafted. Batch: safeoutputs-reliability (related to #48186, #48099).

Generated by 🔧 PR Triage Agent · sonnet46 · 34.1 AIC · ⌖ 7.94 AIC · ⊞ 5.7K · ◷