Remote triggers fail silently when git_repository source is a private repo (personal GitHub account)

Description

Remote triggers (scheduled agents) fail silently when configured with a git_repository source pointing to a private repo on a personal GitHub account (not an org).

Steps to Reproduce

1. Install Claude GitHub App on personal GitHub account
2. Grant app access to a private repo (confirmed visible in repo Settings > Installations)
3. Create a trigger with sources: [{"git_repository": {"url": "https://github.com/<user>/<private-repo>"}}]
4. Fire the trigger via cron schedule or manual run API

Expected Behavior

Agent clones the repo and executes the prompt.

Actual Behavior

• Cron-triggered runs: Trigger fires (metadata shows updated_at and next_run_at advance), but no session is created. No output visible at claude.ai/code/scheduled. Silent failure.
• Manual run via API: Returns HTTP 500 with generic {"error": {"message": "An internal server error occurred", "type": "api_error"}}. No useful error detail.
• No-repo triggers work fine: Same environment, same model, same tools -- removing the git_repository source makes the trigger execute successfully.

Debugging Done

• Confirmed Claude GitHub App is installed on the personal account
• Confirmed app has explicit access to the repo (checked via github.com/settings/installations and repo-level settings)
• Tested minimal trigger (just ls) with repo → 500
• Tested minimal trigger without repo → success
• Tested with and without MCP connections → same behavior
• Repo URL is correct: https://github.com/<user>/<private-repo> (SSH remote works locally, HTTPS format used in trigger config)

Environment

• GitHub account type: Personal (not org)
• Repo visibility: Private
• Trigger environment: claude-code-default
• Model: claude-sonnet-4-6
• Multiple trigger IDs tested, all same behavior

Workaround

Running the agent without repo access (no git_repository source). Output stays in session only -- cannot read repo files or push commits.