[DIC-20] transitive-impact-set CLI flag for aragora decay-monitor

[an0mium]

Slice

Exposes the existing compute_decay_impact_set (with transitive=True) through the aragora decay-monitor CLI by collecting loaded ProofCarryingCodeUnit objects, building a ProofUnitConstraintGraph, and emitting the impacted unit IDs when any claim decay is detected. The result appears under "transitive_impact_set" in JSON output and as a "Transitive impact" section in text output.

Gating

• Default: OFF — transitive_impact flag must be set explicitly in the argparse.Namespace; the underlying ARAGORA_DECAY_MONITOR_ENABLED env gate continues to guard the entire command
• Live queue effect: none — report-only; no state mutation, no issue creation
• Advances issue: [DIC-20] Add epistemic decay monitor for proof-carrying code #6031 (DIC-20)

Tests

• test_transitive_impact_off_by_default_no_key_in_json — JSON output lacks transitive_impact_set without the flag
• test_transitive_impact_no_failures_no_key_in_json — flag on but no failing claims → key absent (no false positives)
• test_transitive_impact_failing_claim_includes_unit_id — failed claim marks its owning unit in the impact set
• test_transitive_impact_stale_claim_included — stale claims also contribute (same reason classes as evaluate_unit)
• test_transitive_impact_result_sorted — two units sharing a failing claim both appear; list is deterministically sorted
• test_transitive_impact_text_output_contains_header — text path prints the header and unit ID

All 22 tests in tests/cli/test_dic20_decay_monitor.py pass (6 pre-existing + 6 new transitive-impact tests + the unchanged pyyaml/flag tests).

Validation

• pytest tests/cli/test_dic20_decay_monitor.py --noconftest — 22 passed
• ruff check aragora/cli/commands/dic20_decay_monitor.py tests/cli/test_dic20_decay_monitor.py — clean
• mypy aragora/cli/commands/dic20_decay_monitor.py --ignore-missing-imports — clean

Out of scope

• Multi-hop dependency edges from manifests (no YAML field for edges yet; ProofUnitConstraintGraph constructed without dependency_edges, so transitive walk equals single-hop — backward-compatible)
• Wiring --transitive-impact into the top-level argparse parser (the namespace flag is consumed; argparse registration is a follow-up CLI-parser slice)
• DIC-21/22 quarantine and repair paths that act on the impact set

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