Remove duplicated user messages #156

Several user-facing messages repeat another message, or sit behind a check an earlier check already makes. Each copy is code to review and keep in sync.

Clear duplicates (first PR):

• create: "Only Bitcoin master-seed backups can be created." _creation_header only returns ms, and rejects other prefixes with "The set header must begin with ms1."
• create: "Use --existing when supplying a seed or secret." This can't be reached, because create already requires an interactive terminal.
• create: HeaderCollision appends "; choose another set header" to the library's "new share set must use a different set header".
• Wallet commands: "Wallet commands accept only Bitcoin master-seed secrets." _parse already rejects anything that isn't ms, and an ms secret is always a MasterSeed.
• Bitcoin Core wallet choice: the "Use blank wallet …? [y/N]" prompt and its y/yes check are spelled out three times.

Needs a decision:

• _FRIENDLY_SET_ERRORS rewords library errors for interactive entry, while piped input shows the library text for the same mistake. Dropping it would surface terse internal text such as "ms and bip39_12w cannot be combined" and "invalid Long codex32 checksum". The single-copy fix is to move the friendly wording into the library. That changes recovery-code messages, so it needs a security review. Two entries are wrong: MismatchedThreshold says "different numbers of shares" for a threshold-0 secret entered into share, and SecretInRecoverySet can't be reached.

Share generation (needs a security review):

• generation._identifier repeats Header's identifier checks.
• generation._seed_input repeats MasterSeed.from_seed's length check.

Recovery code (needs a security review):

• bip93 "interpolation did not produce a secret" appears twice, and the second check is covered by the basis_secret_type check after it.

Smaller repeats (about 1–2 lines each):

• "No unique correction found…" in create --existing vs NO_CORRECTION.
• "Enter one of the displayed numbers.", "That wallet is no longer eligible. Choose again." and "Waiting; press Ctrl-C to stop." are each written twice in _bitcoin_core.py.