fix(watch): keep update outputs project-relative

[takeItIzzy]

Summary

Fixes #433.

graphify update should keep generated paths project-relative for code-only rebuilds, matching the rest of the pipeline.

Root cause

graphify.watch._rebuild_code() resolved the requested path before detection and report generation. That changed a relative invocation such as graphify update . into an absolute rebuild root, which then leaked absolute paths into:

• graphify-out/graph.json source_file fields
• graphify-out/GRAPH_REPORT.md title and path-bearing sections
• graphify-out/graph.html title and node metadata
• graphify-out/cache/*.json AST cache entries

What changed

• keep the cache root absolute, but run detection against the user-provided path so extractor inputs stay project-relative for normal graphify update . usage
• normalize merged rebuild payloads back to project-relative source_file values before report/JSON/HTML export
• render a non-absolute report root label for the common . case
• add watch rebuild regression tests for repo-root and subdirectory invocations

Validation

• .venv/bin/python -m pytest -q tests/test_watch.py
• .venv/bin/python -m pytest -q tests/test_pipeline.py
• manual smoke test: .venv/bin/python -m graphify update . in a temporary repo, verifying no absolute project paths in graph.json, GRAPH_REPORT.md, graph.html, or graphify-out/cache/*.json