feat: Add copy-to-top and copy-to-bottom buffer methods with tests

Add two new methods to copy buffer content regions:
- cj/copy-to-bottom-of-buffer: copies from point to end of buffer
- cj/copy-to-top-of-buffer: copies from beginning of buffer to point

Reorganize copy operations under C-; b c submenu:
- C-; b c w: copy whole buffer
- C-; b c t: copy to top (beginning to point)
- C-; b c b: copy to bottom (point to end)

Other copy operations (buffer name, file path, file link) remain at
C-; b base level for quick access.

Add comprehensive ERT tests (22 tests total):
- test-custom-buffer-file-copy-to-bottom-of-buffer.el (11 tests)
- test-custom-buffer-file-copy-to-top-of-buffer.el (11 tests)
- Tests cover normal, boundary, and error cases including unicode,
  narrowing, whitespace, and read-only buffers

All tests pass.

0 files changed, 0 insertions, 0 deletions