v0.15.1

chore(release): bump workspace to 0.15.1

Patch release: cargo-semver-checks reports no semver update required
against v0.15.0 (196 checks pass; fixes, internal perf, and one additive
public function only). Rolls the Unreleased changelog into 0.15.1 and
updates internal dependency version constraints.

Highlights: qualified-call resolution fix for sem impact recall, cloud
correctness gates (--no-cache and --file now always compute locally),
relative-worktrees support (#341, thanks @zerone0x and @tomasr), faster
graph hydrate on large repos, and the entity-level PR diff GitHub Action.