fix(cli): accept npub on --pubkey identity flags

[nawazish2]

Summary

--mention already accepts hex or npub, but every --pubkey flag only took 64-char hex via validate_hex64. That bites when you copy an npub out of the desktop UI (the usual onboarding path for channels add-member).

Fixes #5167

What changed

• Added normalize_pubkey in buzz-cli — parses with nostr::PublicKey::parse, returns lowercase hex
• Wired it up for identity flags on:
  • channels add-member / remove-member
  • users get / presence
  • dms open / add-member
  • moderation ban / unban / timeout / untimeout
  • agents archive / unarchive (and --replaced-by)
• Left git-protocol pubkeys alone (repo owner, patch/PR author, etc.) — those still expect hex from CLI output
• Updated help text so the flags say "hex or npub"

Test plan

• cargo test -p buzz-cli --lib validate:: (includes hex, uppercase hex, npub, trim, bad input)
• cargo clippy -p buzz-cli --all-targets -- -D warnings
• Manual: buzz channels add-member --channel <uuid> --pubkey npub1... --role member should no longer reject the npub at validation

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