chore: fix clippy lints from newer toolchains

[zawakin]

Why

mise run verify fails on local toolchains newer than the pinned 1.90.0 (e.g. mise's cargo 1.93) because recent clippy versions promoted new cases of collapsible_match and unnecessary_sort_by, and verify runs clippy with -D warnings. These 10 findings are pre-existing on main and unrelated to any recent change.

What

All fixes are mechanical and behavior-preserving:

• collapsible_match — collapse match arms containing a lone if into guarded arms (watch_tui.rs, engine/export.rs, codex/io.rs). The non-matching cases fall through to the existing _ => {} arms, so behavior is identical.
• unnecessary_sort_by — rewrite sort_by(|a, b| ...cmp...) as sort_by_key, using std::cmp::Reverse for the descending sorts (runtime/streamer.rs, storage/repository.rs, presenters/project.rs, 3 SDK examples). sort_by_key is also a stable sort, so same-timestamp event ordering is preserved.
• One test assertion simplification in agtrace-core suggested by clippy.

Verification

• mise run verify (fmt + clippy -D warnings + test + build) now passes locally on cargo 1.93; clippy warning count is 0 across the workspace including --all-targets.
• All tests pass.

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