feat: cora affected + index --watch + call graph wiring (#267, #269)

[ajianaz]

Three features in one PR

Call graph edge extraction (wiring)

Made cora callers and cora impact actually work by extracting call edges during indexing.

• extract_calls() in index/extract.rs: scope tracking + call site extraction
• Reuses engine/context/extraction.rs for symbol reference detection
• detect_function_entry() for Rust/Go/C/Java/Python
• is_builtin() filter (skips 40+ stdlib names)
• Edges stored in call_graph table during index_file()

Runtime verified:

cora callers execute_commit → 1 caller (main)
cora impact open_index --depth 2 → 5 affected sites at depth 1

#267 — cora affected

Find test files affected by source code changes.

cora affected src/main.rs src/index/mod.rs
git diff --name-only | cora affected --stdin
cora affected --json

Two strategies:

1. Call graph: changed symbols → find callers in test files
2. Naming convention: foo.rs → foo_test.rs, tests/foo.rs, foo.test.ts

Runtime verified:

echo "src/main.rs" | cora affected --stdin
→ tests/cli_basic.rs, tests/config_loading.rs

#269 — cora index --watch

Auto-sync file watcher (poll-based, no extra deps).

cora index --watch
# 🔍 Initial index... 1349 symbols
# ✅ Watching for changes... (Ctrl+C to stop)
# 🔄 Updated 2 files, 15 symbols

Tests

• 552 total, clippy clean, fmt clean
• No new deps (reuses existing crates)