docs(parallel-issues): document post-squash-merge conflicts

[thewrz]

This was written agentically; verify its assertions:

Why

Stacked pull requests can conflict after a predecessor is squash-merged even when the successor already carries the predecessor’s content. The workflow needs a content-evidence procedure so an agent does not duplicate changes by choosing a conflict side blindly.

What

• Document why squash merges create history-free merge-down conflicts.
• Require complete blob comparison and an explicit branch-superset finding before resolving.
• Cross-link the procedure from pr-to-green’s serialized auto-merge flow.
• Pin the guidance with reference-manifest assertions.

Decisions

• Keep conflict resolution a reported judgment call; no automatic resolver is introduced.
• Treat the conflict as expected once per squash-merged chain link, while requiring content evidence for the chosen repair.

base=origin/main
files=3
total.insertions=49
total.deletions=1
total.lines=50
operational.files=3
operational.insertions=49
operational.deletions=1
operational.lines=50
generated.files=0
generated.insertions=0
generated.deletions=0
generated.lines=0
lockfile.files=0
lockfile.insertions=0
lockfile.deletions=0
lockfile.lines=0
fixture.files=0
fixture.insertions=0
fixture.deletions=0
fixture.lines=0
non_operational.files=0
non_operational.insertions=0
non_operational.deletions=0
non_operational.lines=0

Testing

• Focused reference-manifest suite passed: .agent/logs/20260824T144714Z-test.log.
• Full suite is blocked by six pre-existing test-agent-preflight.sh cache assertions; reproduced unchanged at base 2f7509e in .agent/logs/20260824T145154Z-test.log.

🤖 Co-authored by Codex gpt-5.6-sol.

Closes #425

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[thewrz]

This was written agentically; verify its assertions:

Adversarial review receipt

• Reviewer: provider=anthropic; model=claude-opus-5; effort=high; mode=cross-provider (reason: peer-cli=claude present; root-owned auto-review launch)
• Counts: P1=0; P2=0; total=0

🤖 Co-authored by Codex gpt-5.6-sol.