Implementation Plan: Keep _EXEMPT_FILES Accurate and Self-Documenting After P3 Migration (#3990)

## Summary

Remove 3 stale entries from `_EXEMPT_FILES` in
`test_no_backend_name_bypass.py` and add self-documenting inline
comments to the remaining 6 entries explaining why each structural
backend-name comparison is unavoidable.

The 3 entries to remove (`execution/backends/__init__.py`,
`cli/_marketplace.py`, `recipe/rules/rules_backend_compat.py`) have zero
AST `Compare` violations — the exemptions were either always technically
incorrect (dict-key lookup, not a Compare node) or became stale after P3
migration refactored comparisons to capability checks.

The 6 remaining entries each have genuine `ast.Compare` nodes
referencing backend-name literals or constants, and each has a
documented reason why the comparison cannot be replaced with a
`BackendCapabilities` field.

## Implementation Plan

Plan file:
`/home/talon/projects/autoskillit-runs/impl-20260610-112706-148380/.autoskillit/temp/make-plan/keep_exempt_files_accurate_plan_2026-06-10_113600.md`

Closes #3933

🤖 Generated with [Claude Code](https://claude.com/claude-code) via
AutoSkillit
<!-- autoskillit:pipeline-signature
steps=prepare_pr,run_arch_lenses,compose_pr,annotate_pr_diff,review_pr
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## Token Usage Summary

| Step | Model | count | uncached | output | cache_read | peak_ctx |
turns | cache_write | time |

|------|-------|-------|----------|--------|------------|----------|-------|-------------|------|
| plan* | opus[1m] | 1 | 898 | 5.9k | 646.5k | 70.6k | 32 | 51.6k | 4m
36s |
| verify* | sonnet | 1 | 60 | 9.5k | 269.5k | 52.5k | 22 | 31.6k | 3m
49s |
| implement* | MiniMax-M3 | 1 | 774.7k | 3.3k | 0 | 0 | 34 | 0 | 7m 4s |
| audit_impl* | sonnet | 1 | 52 | 3.6k | 201.3k | 39.1k | 12 | 21.1k |
2m 3s |
| prepare_pr* | MiniMax-M3 | 1 | 228.1k | 1.9k | 0 | 0 | 16 | 0 | 45s |
| compose_pr* | MiniMax-M3 | 1 | 177.3k | 1.9k | 0 | 0 | 11 | 0 | 44s |
| **Total** | | | 1.2M | 26.0k | 1.1M | 70.6k | | 104.2k | 19m 1s |

\* *Step used a non-Anthropic provider; caching behavior may differ.*

## Token Efficiency

| Step | LoC Changed | cache_read/LoC | cache_write/LoC | output/LoC |
|------|-------------|----------------|-----------------|------------|
| plan | 0 | — | — | — |
| verify | 0 | — | — | — |
| implement | 21 | 0.0 | 0.0 | 155.1 |
| audit_impl | 0 | — | — | — |
| prepare_pr | 0 | — | — | — |
| compose_pr | 0 | — | — | — |
| **Total** | **21** | 53204.3 | 4963.4 | 1239.4 |

## Model Usage Breakdown

| Model | steps | uncached | output | cache_read | cache_write | time |
|-------|-------|----------|--------|------------|-------------|------|
| opus[1m] | 1 | 898 | 5.9k | 646.5k | 51.6k | 4m 36s |
| sonnet | 2 | 112 | 13.1k | 470.8k | 52.6k | 5m 52s |
| MiniMax-M3 | 3 | 1.2M | 7.0k | 0 | 0 | 8m 33s |

Author: Trecek

tests/arch/test_no_backend_name_bypass.py

@@ -9,15 +9,18 @@
 # Exempt files where backend name comparison is structurally necessary.
 _EXEMPT_FILES: frozenset[str] = frozenset(
     {
-        "execution/backends/__init__.py",  # BACKEND_REGISTRY lookup
-        "server/_factory.py",  # Feature-gated backend swap (pre-capabilities)
-        "cli/_marketplace.py",  # Marketplace — Claude Code-only install guard
-        "execution/recording.py",  # Replay setup: fmt == "codex" Compare for player selection
-        "execution/headless/_headless_result.py",  # Claude-specific result parsing
-        "core/_version_snapshot.py",  # Version snapshot — routes codex_version by backend name
-        "execution/headless/_headless_helpers.py",  # assert_headless_cmd claude -p flag check
-        "cli/session/_session_launch.py",  # Feature-gate backend-alignment fallback
-        "recipe/rules/rules_backend_compat.py",  # Backend compatibility semantic rule
+        # Pre-instantiation config guard: backend object not yet constructed
+        "server/_factory.py",
+        # Cassette format from filenames; selects api_simulator player (no backend at replay time)
+        "execution/recording.py",
+        # Claude-specific JSONL stdout format; parse_session_result() is Claude-only
+        "execution/headless/_headless_result.py",
+        # IL-0 module: cannot import BackendCapabilities (IL-1); routes version data by name
+        "core/_version_snapshot.py",
+        # Binary name check on CmdSpec subprocess argv; no backend context in assert_headless_cmd()
+        "execution/headless/_headless_helpers.py",
+        # FeatureDef.requires_backend_alignment is config-layer; no capabilities at scan time
+        "cli/session/_session_launch.py",
     }
 )