docs: make the README describe the script that exists

With the simulation gate implemented, the safety model section is now
true, and says where the gate runs (inside convert, immediately before
the install) and what it writes. Plan mode is labelled advisory, because
that is what it is: it logs a simulation for a human to read and decides
nothing.

Removed four options the README documented that were never implemented:
--no-auto-fix, --overwrite-keyring, --recreate-keyring and
--no-purge-flavor. Documented --preserve-snap, which was implemented and
missing. Every flag now matches the parser in both directions.

Two troubleshooting sections described behaviour that does not exist.
Keyring recovery pointed at the two missing keyring flags; the real
mechanism is a check against the expected Mint key ID with --yes to
overwrite, which is worth stating since it is what makes the plain-HTTP
fallback safe. Flavor-package purging was never written, so the login
loop it referred to is now described as something to fix by hand.

Also removes --no-install-recommends from the usage text, which I added
in the previous commit believing it was a script flag. It is an apt
option inside apt_opts_common; the parser rejects it. That is the same
class of error this commit exists to fix, found by checking the flag
lists against the parser in both directions rather than trusting either.

ubuntu-to-mint-convert-v3.sh

@@ -169,7 +169,6 @@ Options:
   --keep-ppas                      Do not disable third-party sources (not recommended)
   --preserve-snap                  Keep snapd (default: enabled)
   --with-recommends                Allow recommended packages (default: off)
-  --no-install-recommends          Force --no-install-recommends (this is the default)
   --max-removals N                 Abort if the simulation removes more than N
                                    packages (default: ${MAX_REMOVALS})
   --yes                            Non-interactive / auto-confirm