Adopt cargo clippy as a CI gate (#2096 follow-up)

[carlos-alm]

Found while working on

#2096 (Rust codebase had drifted from cargo fmt/clippy conventions). That issue's cargo fmt half is fixed (one-time repo-wide reformat + cargo fmt --check CI gate). This issue tracks the remaining cargo clippy half.

Context

As of #2096's investigation, cargo clippy reports 98 warnings against crates/codegraph-core, and nothing in CI runs clippy today.

Unlike cargo fmt (a purely mechanical, semantics-preserving reformat — safe to apply repo-wide in one automated pass, which #2096 already did), clippy warnings often require actual judgment per-lint: some are genuine simplifications, some flag idiomatic-but-flagged patterns, and a few can be legitimate false positives worth an #[allow(...)] with a comment rather than a code change. Blindly "fixing" all 98 in one pass risks introducing subtle behavior changes without adequate review, so it wasn't folded into #2096.

Suggested approach

1. Run cargo clippy --workspace and triage the 98 warnings by lint category — likely a handful of #[allow]-worthy categories dominate the count rather than 98 independent issues.
2. Fix (or explicitly allow with justification) each category, verifying behavior via cargo test after each batch — probably worth splitting into a few smaller PRs by category rather than one large diff, given the review burden the count implies.
3. Add cargo clippy --workspace -- -D warnings (or a more targeted lint-level gate) to the existing rust-check CI job once the crate is clean, mirroring how Rust codebase has drifted from cargo fmt/clippy conventions (~1152 diff hunks, 98 warnings), not CI-enforced #2096 added cargo fmt --check to the same job.