fix: clippy unnecessary_sort_by in cognitive_memory blocking PR #889 CI (unblocks #890)

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Context

PR #889 (research idea extractor) merged but the verify workflow is failing on cargo clippy --all-targets --all-features --locked -- -D warnings due to clippy::unnecessary_sort_by lints introduced/exposed by the merge. This blocks validation issue #890 which gates the end-to-end smoke of the idea extractor and therefore blocks closure of goal explore-developer-ideas-from-tracked-researchers.

Failing locations

From CI log of run 24597973906 and local grep:

• src/cognitive_memory/mod.rs:316 — candidates.sort_by(|a, b| b.0.cmp(&a.0));
• src/cognitive_memory/mod.rs:549 — backups.sort_by(|a, b| b.0.cmp(&a.0));
• src/knowledge_context.rs:63 — scored.sort_by(|a, b| b.0.cmp(&a.0));

(Verify with full clippy run; there may be more.)

Fix

Replace each with sort_by_key + Reverse per clippy hint:

use std::cmp::Reverse;
candidates.sort_by_key(|x| Reverse(x.0));

Acceptance

• cargo clippy --all-targets --all-features --locked -- -D warnings passes locally and in CI
• cargo test --lib passes (no behavior change)
• verify workflow green on main
• Comment on validate: end-to-end run of newly-merged research idea extractor (#881) #890 noting CI is unblocked so validation can proceed