fix(core): deduplicate denied secret prompts in orchestration loop

[bug-ops]

Summary

• Closes fix(orchestration): duplicate secret prompt shown after 120s timeout in plan confirm #1455
• process_pending_secret_requests() now accepts a plan-scoped denied: &mut HashSet<(String, String)> tracking (handle_id, secret_key) pairs already denied in this plan execution
• On timeout or explicit user rejection the pair is inserted into denied; subsequent re-requests from the same sub-agent for the same key are auto-denied with a tracing::debug! log without re-prompting the user
• Approved secrets are not tracked — re-requesting an approved key after TTL expiry prompts normally
• Different sub-agents requesting the same key are handled independently via the composite (handle_id, key) pair
• The HashSet is local to run_scheduler_loop() and is dropped when the plan ends; new plans start fresh

Test plan

• All existing tests pass (cargo nextest run --workspace --features full --lib --bins — 4917 passed)
• cargo clippy --workspace --features full -- -D warnings — clean
• cargo +nightly fmt --check — clean
• Manual: start a plan with a sub-agent that requests a secret, let the prompt time out, verify the prompt does not reappear on subsequent ticks