fix(rlm): add SAFETY comment to production set_var in auto_configure_llm

Resolves the P3-1 finding from #2797 (security-sentinel 2026-06-20 scan):
`crates/terraphim_rlm/src/rlm.rs:942` called `unsafe { std::env::set_var(...) }`
in a non-test auto-configure path with no `// SAFETY:` justification, unlike the
established pattern in `terraphim_spawner/src/lib.rs` (pre_exec setrlimit).

Rust 2024 made `set_var` unsafe because env vars are process-global and mutating
one races concurrent readers. Document the invariant that makes this call sound:
`auto_configure_llm` runs once at RLM startup initialisation, before any worker
tasks or child processes are spawned, so there are no concurrent readers of
`OPENROUTER_API_KEY` at the call site. The key value originates from a trusted
source (env var or 1Password), not untrusted input.

Comment-only change: zero behavioural change, zero new code. Surgical scope per
the issue's P3-1 remediation note ("add SAFETY justification" — moving to a
config struct is explicitly the alternative, not required).

Verification (this session):
- cargo check  -p terraphim_rlm                     -> clean (5.30s)
- cargo test   -p terraphim_rlm --lib               -> 135 passed, 0 failed
- cargo fmt    -p terraphim_rlm --check on this hunk -> clean (the crate-wide
  fmt drift reported is pre-existing on origin/main, unrelated to this comment)
- cargo clippy -p terraphim_rlm --all-targets        -> the 3 errors
  (2x doc-list-indent, 1x redundant_closure) are PRE-EXISTING on origin/main
  (verified by stashing this edit and re-running on pristine HEAD); this
  comment-only change introduces zero new lint findings
- ubs crates/terraphim_rlm/src/rlm.rs                -> 0 critical, 0 warning

The P1-1 (LLM proxy 0.0.0.0:3456) and P1-2 (git2 unsound) findings from #2797
are host-infra / upstream respectively and out of scope for this surgical fix;
P1-1 requires human root on bigbox (fleet agents blocked by no_new_privs per
the recurring sentinel comments).

Refs #2797

Co-Authored-By: Claude <noreply@anthropic.com>

Author: Test User

crates/terraphim_rlm/src/rlm.rs

@@ -938,7 +938,16 @@ impl TerraphimRlm {
                 .unwrap_or_else(|_| "meta-llama/llama-3.2-3b-instruct:free".to_string());
             role.llm_api_key = Some(key.clone());
             role.llm_model = Some(or_model.clone());
-            // Cache for child processes and build_llm_from_role
+            // Cache for child processes and build_llm_from_role.
+            //
+            // SAFETY: `set_var` is only unsafe in Rust 2024 because env vars are
+            // process-global and mutating one can race concurrent readers. This
+            // runs once during `auto_configure_llm` (RLM startup initialisation),
+            // before any worker tasks or child processes are spawned, so there are
+            // no concurrent readers of `OPENROUTER_API_KEY` at this point. The
+            // value is propagated to spawned processes intentionally (see comment
+            // above) and the key originates from a trusted source (env var or
+            // 1Password), not untrusted input.
             unsafe {
                 std::env::set_var("OPENROUTER_API_KEY", key);
             }