feat: add headless browser rendering for JavaScript SPA sites (#321)

New BrowserRenderer class uses Playwright to render JavaScript-heavy
documentation sites (React, Vue SPAs) that return empty HTML shells
with requests.get(). Activated via --browser flag on web scraping.

- browser_renderer.py: Playwright wrapper with lazy browser launch,
  auto-install Chromium on first use, context manager support
- doc_scraper.py: browser_mode config, _render_with_browser() helper,
  integrated into scrape_page() and scrape_page_async()
- SPA detection warnings now suggest --browser flag
- Optional dep: pip install "skill-seekers[browser]"
- 14 real e2e tests (actual Chromium, no mocks)
- UML updated: Scrapers class diagram (BrowserRenderer + dependency),
  Parsers (DoctorParser), Utilities (Doctor), Components, and new
  Browser Rendering sequence diagram (#20)

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

CHANGELOG.md

@@ -8,6 +8,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 ## [Unreleased]
 
 ### Added
+- **Headless browser rendering** (`--browser` flag) — uses Playwright to render JavaScript SPA sites (React, Vue, etc.) that return empty HTML shells. Auto-installs Chromium on first use. Optional dep: `pip install "skill-seekers[browser]"` (#321)
 - **`skill-seekers doctor` command** — 8 diagnostic checks (Python version, package install, git, core/optional deps, API keys, MCP server, output dir) with pass/warn/fail status and `--verbose` flag (#316)
 - **Prompt injection check workflow** — bundled `prompt-injection-check` workflow scans scraped content for injection patterns (role assumption, instruction overrides, delimiter injection, hidden instructions). Added as first stage in `default` and `security-focus` workflows. Flags suspicious content without removing it (#324)
 - **6 behavioral UML diagrams** — 3 sequence (create pipeline, GitHub+C3.x flow, MCP invocation), 2 activity (source detection, enhancement pipeline), 1 component (runtime dependencies with interface contracts)