test(admin): add unit test suite for buzz-admin CLI

[SeanGearin]

Problem

crates/buzz-admin (the operator CLI: membership management, migrations,
channel reconciliation) had zero tests. Its pure logic — role validation,
pubkey parsing/normalization, the NIP-43 same-second-domination timestamp
bump, and the clap argument surface — was only exercised manually against a
live deployment.

Fix

Adds a #[cfg(test)] unit test module (no Postgres, no Redis, no network)
covering:

• clap definition — Cli::command().debug_assert() (same pattern as
  buzz-cli's cli_definition_is_valid), plus parse tests for every
  subcommand's argument surface: add-member role default + required
  --pubkey, remove-member optional role filter, product-feedback list
  limit default (100) and 1..=1000 range enforcement, reconcile-channels
  optional --relay-key, unknown-subcommand rejection.
• validate_role — accepts member/admin; rejects owner with the
  RELAY_OWNER_PUBKEY hint; rejects unknown/case-mismatched roles with the
  offending role named in the error.
• parse_pubkey_hex — hex round-trip, npub decoding, uppercase-hex
  normalization to lowercase, garbage and truncated-hex rejection (error
  echoes the input). Fixtures are derived from a fixed secret key at test
  time, so no hard-coded pubkey constants can drift.
• bumped_created_at — the kind:13534 same-second domination guard
  documented in the module header. The max(now, newest_existing + 1s)
  expression is extracted from publish_membership_list_with_bump into a
  named pure function (byte-identical behavior, same call site) so the four
  cases are testable: no existing list, older existing, same-second
  collision, and backwards clock skew.

The only production-code change is that six-line extraction; no behavior
changes.

The suite is wired into just test-unit (both the cargo-nextest branch and
the scripts/run-tests.sh fallback), so it gates CI's unit job and the
lefthook pre-push hook rather than running only on a manual
cargo test -p buzz-admin. Happy to drop those two lines and leave the
wiring to a follow-up if you'd rather keep this PR test-only.

Test evidence

cargo fmt -p buzz-admin -- --check          # clean
cargo clippy -p buzz-admin --all-targets -- -D warnings   # clean
cargo test -p buzz-admin                    # 21 passed; 0 failed
just test-unit                              # fallback path; "buzz-admin unit
                                            # tests" step runs the 21 tests

(cargo-nextest is not installed in this environment, so just test-unit
exercised the run-tests.sh fallback branch end-to-end; the nextest line
mirrors the existing per-package invocations.)