chore(echidnabot): add spark to secondary_languages (#294)

Adds `spark` to `.echidnabot.toml` secondary_languages per the standing
estate rust/SPARK pairing rule. Audit note: SPARK was never present in
this list (original omission, not bot drift — verified via `git log
-S`). Part of 2026-07-01 completion-plan Phase 0.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-authored-by: Claude Fable 5 <noreply@anthropic.com>

Author: hyperpolymath

.echidnabot.toml

@@ -12,7 +12,7 @@ type = "formal-verification"
 [architecture]
 enforcement_level = "normal"
 primary_language = "rust"    # Core prover backends
-secondary_languages = ["julia", "rescript", "idris2", "chapel"]
+secondary_languages = ["julia", "rescript", "idris2", "chapel", "spark"]
 
 [provers]
 # ECHIDNA is self-hosting - can verify its own Idris2 ABI definitions