release: v0.8.15 — fix planning-docs CI path and refresh public discovery docs

[imaurer]

Summary

• Bumps version to 0.8.15 in pyproject.toml, Cargo.toml, uv.lock, and Cargo.lock
• Prepends 0.8.15 changelog entry dated 2026-03-11
• Ships the planning-docs CI path fix from PR Fix CI planning-doc contract path assumption #191 (fixes the FileNotFoundError that broke v0.8.14 release validation)
• Ships the public discovery docs refresh from PR docs: teach search all as the unified cross-entity discovery entry point #190 (search all as unified entry point)
• Updates analysis/technical/overview.md current-version line and tightens tests/test_docs_changelog_refresh.py assertions

Test plan

• ./scripts/check-version-sync.sh passes (Versions in sync: 0.8.15)
• cargo fmt --check passes
• cargo clippy -- -D warnings passes
• cargo test passes (570 tests)
• pytest tests/ passes (33 tests)
• mkdocs build --strict passes
• git merge-base --is-ancestor 6dcfec5 HEAD confirms PR Fix CI planning-doc contract path assumption #191 fix is included