Add a hermetic Homeboy test context

[chubes4]

Summary

• Finalized Homeboy agent-task cook run recovery-homeboy-8150-hermetic-rust-tests into review-ready branch fix/8150-hermetic-rust-tests.

Proof provenance

• Proof runner: Homeboy agent-task cook loop
• Proof ID: agent-task-finalization:recovery-homeboy-8150-hermetic-rust-tests
• Homeboy run ID: recovery-homeboy-8150-hermetic-rust-tests
• Manual proof: none recorded by this Homeboy finalization

Publication intent

• Schema: homeboy/agent-task-publication-intent/v1
• Action: review_request
• Target kind: code_review
• Adapter: github_pull_request
• Base: main
• Head: fix/8150-hermetic-rust-tests

Source refs

• Make the Homeboy Rust test suite hermetic from host runtime state #8150

Source relationship

• Related finding ID: none recorded
• Source packet ID: none recorded
• Change kind: test-only
• Supersedes: none recorded
• Depends on: none recorded

Runtime guardrails

• Why this is not broader than the packet evidence: Migrates one coherent high-failure family first; remaining ambient-state families stay listed for bounded follow-up rather than a risky repository-wide rewrite.
• Evidence discriminators preserved: explicit test-owned state roots and Cargo fixture binary
• Nearby contracts preserved: legacy HomeGuard serialized fallback, host integration remains explicit opt-in

Attempt summary

Introduces an explicit subprocess-scoped test context for HOME/config/data/artifact/runtime/temp/daemon/runner paths and migrates the stale linked-rig integration family.

Gate results

• hermetic-integration-parallel: passed (targeted)
• hermetic-integration-serial: passed (targeted)
• artifact-path: passed (targeted)
• format: passed (targeted)
• diff-check: passed (targeted)

Verification capability

• targeted_checks_run: cargo test --test stale_linked_rig_recovery --no-fail-fast, cargo test --test stale_linked_rig_recovery -- --test-threads=1, cargo test --lib core::paths::tests::artifact_root_defaults_under_homeboy_data, cargo fmt --check, git diff --check
• targeted_checks_unavailable: none recorded
• ci_expected: none recorded
• manual_reviewer_check: none recorded

CI-equivalent coverage

• CI-equivalent required gate: CI-equivalent required gate was not recorded; targeted proof must not be treated as full CI coverage

Changed files

• docs/internals/test-tiers.md
• src/lib.rs
• src/test_support.rs
• tests/stale_linked_rig_recovery.rs

Artifact refs

• none recorded

Final status

• Status: review-ready
• Base: main
• Head: fix/8150-hermetic-rust-tests
• Merge/deploy: not performed

AI assistance

• AI assistance: Yes
• Tool(s): OpenCode
• Model: openai/gpt-5.6-sol
• Used for: Compared parallel and serial host-state failures, implemented the shared hermetic context and first migration slice, and verified the candidate with Chris.