Fix worktree cleanliness and no-checkout handling

[mizchi]

Summary

• preserve executable modes when adding linked worktrees
• make status and non-force removal consistently detect mode-only changes
• compare symbolic links by link target and avoid following them during cleanup
• implement worktree add --no-checkout without materializing files or an index
• ensure whitebox tests use the build variant matching the test runner

Root cause

Worktree checkout initially materialized blobs as regular files, while status and removal used different comparison paths. Status followed symbolic links, removal compared only file bytes, and the parsed --no-checkout flag was not forwarded to the worktree creation layer.

Validation

• pkf run build
• worktree whitebox tests: 7/7 passed
• mizchi/bit_runtime: 14/14 passed
• mizchi/bit_lib: 302/302 passed
• Git worktree-only suites: t2401–t2407 passed; the targeted t2400 --no-checkout case now passes
• moon check --target native modules/bit modules/bit_lib modules/bit_runtime
• package layering check and git diff --check
• manual release-binary checks for executable mode, symlinks, dirty removal, and no-checkout/reset behavior

Closes #172