fix(ci): fetch enough upstream history for the fork stack check

[lukemaj]

Problem

The Fork Stack Model job fetches upstream main --depth 100. Upstream pingdotgg/t3code main is now 113 commits past the fork's base (b2b43be), so the shallow fetch does not contain the merge base. git merge-base HEAD upstream/main fails under set -e and scripts/fork-check.sh exits 1 with no message. Every fork PR fails this check (e.g. #38, run 36239606669, and #37).

Fix

• .github/workflows/fork.yml: git fetch --filter=blob:none upstream main (full history, no blobs) instead of --depth 100.
• scripts/fork-check.sh: when the merge-base lookup fails, print fork-check: FAIL: cannot find the merge base with <remote>/main (fetch more history?) to stderr and exit 1.

docs/fork.md does not mention the fetch depth, so no doc change.

Proof

• Worktree with full blobless upstream fetch: bash scripts/fork-check.sh --upstream upstream ends with fork-check: OK. (exit 0).
• Scratch clone with git fetch upstream main --depth 100: prints fork-check: FAIL: cannot find the merge base with upstream/main (fetch more history?), exit 1. After git fetch --filter=blob:none --unshallow upstream main: fork-check: OK., exit 0.

Model: Claude Opus 5.5 (medium) via T3 Code

🤖 Generated with Claude Code

[github-actions]

Thread transfer impact

✅ Thread transfer remains within every enforced ceiling.

Provider	Metric	Main baseline	This PR	Impact	PR ceiling
Codex	Total thread wire	13.5 KiB	13.5 KiB	−55 B (−0.4%)	15.1 KiB	✅
Codex	Thread snapshot wire	7.0 KiB	7.0 KiB	−1 B (−0.0%)	7.3 KiB	✅
Codex	Live turn WebSocket wire	6.5 KiB	6.4 KiB	−54 B (−0.8%)	7.8 KiB	✅
Codex	Live turn WebSocket decoded	56.3 KiB	56.2 KiB	−44 B (−0.1%)	66.4 KiB	✅
Codex	Live turn messages	10	9	−1 (−10.0%)	21	✅
Claude	Total thread wire	13.5 KiB	13.5 KiB	+16 B (+0.1%)	15.1 KiB	✅
Claude	Thread snapshot wire	7.1 KiB	7.1 KiB	+2 B (+0.0%)	7.3 KiB	✅
Claude	Live turn WebSocket wire	6.4 KiB	6.4 KiB	+14 B (+0.2%)	7.8 KiB	✅
Claude	Live turn WebSocket decoded	57.0 KiB	57.0 KiB	0 B (0.0%)	66.4 KiB	✅
Claude	Live turn messages	9	9	0 (0.0%)	21	✅

Baseline: 8b50929 · PR result: 0cc76a5 · Source CI: success

Scenario and decoded snapshot size

10 historical turns, 5 command tools per turn, 878.9 KiB retained MCP result per historical turn, and a 1.05 MiB retained result in the measured turn.

• Codex decoded thread snapshot: 113.9 KiB
• Claude decoded thread snapshot: 114.6 KiB

Updated in place by a trusted workflow. PR artifacts are strictly validated and never executed.