chore: project cleanup — remove legacy, fix gitignore, commit Cargo.lock, README

- Delete legacy/ (retired Python oracle + generators). Nothing in the build or
  tests depends on it; parity runs off committed golden JSON. Drop the dangling
  `see legacy/` note in parity_test.rs.
- .gitignore: rewrite for a pure-Rust project (drop dead Python/Node sections);
  stop ignoring Cargo.lock and commit it for reproducible binary builds; negate
  the ignore for the committed parity fixture lcov.info.
- Track crates/ci-core/tests/fixtures/synthetic_project/lcov.info — previously
  ignored, so the coverage parity test would have failed on a clean checkout/CI.
- Remove the stale docs/session-handoff-phase-5-6.md (work long completed).
- Rewrite README.md with the current architecture: 3-tier resolution, import
  edges, edges_ready gating, opt-in semantic embeddings, and distribution.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

Author: Your Name

.gitignore

@@ -1,46 +1,24 @@
-# Python
-__pycache__/
-*.py[cod]
-*.pyo
-*.pyd
-.Python
-*.egg-info/
-dist/
-build/
-.eggs/
-.venv/
-venv/
-env/
+# Rust build output
+/target/
 
-# SQLite / codeindex DB
+# Index database (per-project, generated)
 .codeindex/
 *.db
 *.db-wal
 *.db-shm
 
-# Coverage
+# Coverage output — but keep the committed parity-test fixture
 .coverage
 coverage.out
 coverage.xml
 lcov.info
 coverage/
 .nyc_output/
+!crates/ci-core/tests/fixtures/synthetic_project/lcov.info
 
-# Node / TypeScript
-node_modules/
-*.js.map
-dist/
-.tsbuildinfo
-
-# OS
+# OS / editor
 .DS_Store
 Thumbs.db
-
-# Rust
-target/
-Cargo.lock
-
-# Editor
 .vscode/
 .idea/
 *.swp