fix: sweep remaining env-mutating unit tests for hermeticity

[BunsDev]

Summary

Follow-up to #440, completing the sweep: zero env_lock/EnvVarGuard/set_var/remove_var remain in coven-cli (grep -rn clean). All conversions use dependency injection; public signatures and production behavior are unchanged.

harness.rs (~45 lock-guarded tests → lock-free)

• New AdapterEnv snapshot (COVEN_HOME, COVEN_HARNESS_ADAPTER_MANIFEST, COVEN_HARNESS_ADAPTER_DIRS) taken once per resolution and threaded through external_adapter_manifest_sources / external_harness_specs / configured-spec helpers; pub fns are thin from_process() wrappers.
• Command construction split: env-free command_parts_with_specs(specs, …, codex_json, claude_bypass_enabled, resolve_program) holds the logic; the inner wrapper reads process env. Dropped the now-redundant with_claude_permission_flags variant.
• harness_available_with(executable, engine_resolves) seam; engine test injects engine::resolve_from.
• Tests inject built-in/external specs, AdapterEnv, and an identity program resolver — side benefit: program == asserts no longer depend on host PATH (notably flaky-prone on Windows).

privacy.rs

• PrivacyEnv snapshot + single apply_env_overrides, deduplicating the copy-pasted override block while preserving the original file → env → settings → env precedence. Added retention-days override coverage (positive, zero, non-numeric).

main.rs → paths.rs

• Moved the USERPROFILE fallback test onto the already-injectable coven_home_from_env seam; deleted the last env_lock/EnvVarGuard helpers from main.rs.

api.rs (latent env-dependent failure found by this sweep)

• launch_request_with_unknown_harness_returns_400_upfront_no_session_row used hermes — a real installable adapter — so on any machine with local adapter recipes (~/.coven/adapters) the launch succeeded (201 ≠ 400). Reproduced on current main; CI only passes because runners are bare. Switched to a synthetic id so the test is hermetic everywhere.

Verification

• cargo fmt --check ✅
• cargo clippy --workspace --all-targets --locked -- -D warnings ✅ (0 warnings)
• cargo test --workspace --locked ✅ 1269 passed / 0 failed
• python scripts/check-secrets.py ✅

Readiness

• Scoped to test hermeticity; no behavior change intended. Riskiest bit is the harness.rs command-construction split — covered by the full existing test suite routed through the new seam.