feat: implement chmod/chown/utimens, fix access perms, add --allow-root

[haoren3696]

I attempted to compile, build, and install ContextFS on the ContextFS file system and encountered the following issue. After troubleshooting, it was confirmed that the current ContextFS is missing the utimens, chmod, and chown FUSE handlers.I believe that as an Agent file system, ContextFS needs to first support compiling, building, and installing ContextFS within the ContextFS file system.

The following error is reported:

[root@LAPTOP-CG8RLAN9 ContextFS]# agentvfs workspace init selfhost --from .
name=selfhost
source=/run/user/0/agentvfs/selfhost/source
seeded_from=.
status=initialized
[root@LAPTOP-CG8RLAN9 ContextFS]# agentvfs workspace start selfhost
warning=no usable telemetry backend found; mounted without telemetry
name=selfhost
mount=/run/user/0/agentvfs/selfhost/mount
socket=/run/user/0/agentvfs/selfhost/control.sock
store=/run/user/0/agentvfs/selfhost/store
telemetry=none
status=started
[root@LAPTOP-CG8RLAN9 ContextFS]# cd /run/user/$(id -u)/agentvfs/selfhost/mount
[root@LAPTOP-CG8RLAN9 mount]# rm -rf build
[root@LAPTOP-CG8RLAN9 mount]# cmake -B build -DAGENTVFS_EBPF=OFF
CMake Error at /usr/share/cmake/Modules/CMakeDetermineSystem.cmake:224 (configure_file):
  Function not implemented
Call Stack (most recent call first):
  CMakeLists.txt:2 (project)


-- The C compiler identification is GNU 12.3.1
CMake Error at /usr/share/cmake/Modules/CMakeDetermineCCompiler.cmake:226 (configure_file):
  Function not implemented
Call Stack (most recent call first):
  CMakeLists.txt:2 (project)


-- The CXX compiler identification is GNU 12.3.1
CMake Error at /usr/share/cmake/Modules/CMakeDetermineCXXCompiler.cmake:232 (configure_file):
  Function not implemented
Call Stack (most recent call first):
  CMakeLists.txt:2 (project)


-- Detecting C compiler ABI info
CMake Error at /usr/share/cmake/Modules/CMakeDetermineCompilerABI.cmake:57 (try_compile):
  Failed to set working directory to
  /run/user/0/agentvfs/selfhost/mount/build/CMakeFiles/CMakeScratch/TryCompile-C3q7JD
  : Permission denied
Call Stack (most recent call first):
  /usr/share/cmake/Modules/CMakeTestCCompiler.cmake:26 (CMAKE_DETERMINE_COMPILER_ABI)
  CMakeLists.txt:2 (project)


-- Configuring incomplete, errors occurred!

Code Modification Summary:

• Add utimens, chmod, chown FUSE handlers to silence ENOSYS on touch, chmod, chown, cp -p, rsync -a
• Fix cas_access to check owner permission bits (0400/0200/0100) instead of other bits (0004/0002/0001), fixing chdir EACCES on owner-only directories
• Add --allow-root flag to 'agentvfs workspace init/start', persisted in workspace.json, passes -o allow_root to the FUSE daemon
• Document self-hosting workflow in README

[Zhihaoi]

LGTM