fix: normalize LLM-emitted scalar shapes on issue data

[AbirAbbas]

Summary

Fixes a build-killing crash found by live-benchmarking swe-planner.build on openrouter/deepseek/deepseek-v4-pro: generate_fix_issues returned fix issues whose acceptance_criteria was a bare string, and 18 minutes later the replanner's DAGState re-validation failed with six list_type errors — a 48-minute build died with zero deliverable (cleanup then deleted the fix branches). Internal trace: exec_20260720_110500_m36lxxcq.

The mechanics: raw issue dicts bypass validation (DAGState.all_issues is list[dict]), and the executor's attribute assignment onto IssueResult.final_acceptance_criteria bypasses Pydantic (validate_assignment is off), so the poison persisted into the checkpoint and only detonated on the next model re-validation. A second identical run happened to get lists from the model and passed — the failure is a coin flip on weak-model output shape.

Fix (both stacks)

• Boundary normalization: str → [str] coercion on issue list fields at every LLM ingestion point — fix-generator output, replanner updated_issues/new_issues (normalize_issue_dict / dagutil.NormalizeIssueDict).
• Tolerant re-validation: mode="before" validators on IssueResult.final_acceptance_criteria, SplitIssueSpec and PlannedIssue list fields; Go StrList type + asStringSlice bare-string case — so pre-fix poisoned checkpoints still load and Go stops silently dropping bare-string values.
• Both executor assignment sites now coerce explicitly.

Test plan

• tests/test_llm_shape_normalization.py — the exact incident as a regression test (checkpoint reload with a poisoned completed_issue), boundary coercion, apply_replan normalization; Go mirrors in internal/dagutil + internal/schemas
• Full suite at CI parity (no .env): 1079 passed, 1 skipped + compileall clean
• Go: gofmt clean, build, vet, go test -race -count=1 ./... with GOWORK=off

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🤖 Generated with Claude Code

[AbirAbbas]

Folded into #103 — same commit (62ffbd0) is already the first commit of that branch, so everything from this round ships as one PR.