Add immediate safety warning to README (#12)

* Add immediate safety warning to README

- Remove the production-use claim from Project Status.
- Mark the project experimental until the P0 data-safety checklist
  passes, with a prominent warning banner up top.
- Add a Limitations section: warn against modifying hardlinked
  snapshots in place, document the lack of a restore command, and
  note special-entry exclusion and symlink-preservation behavior.
- Check off the corresponding IMPROVEMENTS.md acceptance criteria.

* Qualify special-entry exclusion limitation to the Python implementation

--external-rsync uses rsync's --archive flag, which copies/recreates
FIFOs, sockets, and device nodes instead of excluding them - the
exclusion behavior only applies to the default Python implementation.

* Qualify device-node recreation reliability under --external-rsync

rsync -a implies -D (devices/specials), but creating device nodes
typically requires root on the destination - without it rsync may
skip or fail on them rather than reliably recreating them.

---------

Co-authored-by: Maxim Shulga <maxim.shulga@alludo.com>

IMPROVEMENTS.md

@@ -120,10 +120,10 @@ Acceptance criteria:
 
 ### Add an immediate safety warning
 
-- [ ] Remove the current production-use claim from `README.md`.
-- [ ] State that the software is experimental until the P0 release gate passes.
-- [ ] Warn users not to modify hardlinked snapshots in place.
-- [ ] Document current recovery and metadata limitations honestly.
+- [x] Remove the current production-use claim from `README.md`.
+- [x] State that the software is experimental until the P0 release gate passes.
+- [x] Warn users not to modify hardlinked snapshots in place.
+- [x] Document current recovery and metadata limitations honestly.
 
 ## P1 - Recovery and release readiness