Fix git sync and add repo browser with path selection

- Fix discover_files: rel_path always computed (was stuck at '.' at root),
  include_path_match now uses relative path, 'return' changed to 'continue'
- Fix ingest_git_source: files were cloned but ingested from wrong path
  (docs/repo-id instead of data/repos/repo-id). Now stages filtered files
  into DOCS_PATH/library_id before calling ingest_library.
- Add browse_repo_tree() for interactive repo exploration
- Add POST /api/v1/sources/browse endpoint to backend
- Add /sources/browse proxy route to webui
- Rewrite sources.html: browse repo, expand/collapse tree, check paths to
  include, then save source and sync

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

webui/app/main.py

@@ -5,7 +5,7 @@ from pathlib import Path
 from typing import List, Optional
 
 from fastapi import FastAPI, File, Form, Request, UploadFile
-from fastapi.responses import HTMLResponse, RedirectResponse
+from fastapi.responses import HTMLResponse, JSONResponse, RedirectResponse
 from fastapi.staticfiles import StaticFiles
 from fastapi.templating import Jinja2Templates
 
@@ -257,6 +257,19 @@ def parse_path_list(value: str) -> List[str]:
     return paths
 
 
+@app.post("/sources/browse")
+async def browse_source(repo_url: str = Form(...), branch: str = Form("main")):
+    client = get_client()
+    try:
+        result = await client.post(
+            "/api/v1/sources/browse",
+            json={"repo_url": repo_url, "branch": branch},
+        )
+        return JSONResponse(content=result)
+    except Exception as e:
+        return JSONResponse(status_code=400, content={"success": False, "error": str(e)})
+
+
 @app.post("/sources/add")
 async def add_source(
     library_id: str = Form(...),