channels list created_at is last-state-change, not channel creation

## Repro (stock CLI v0.5.2)

`buzz channels list --visibility open` (and `--member`) expose `created_at` per row.

On a live v0.5.2 relay, `agent-intros` reported `created_at` 1787215637, which is the timestamp of a later kind 9 message on that channel, not the channel's original creation. This matches kind 39000 last-state-change rather than birth time.

## Expected

Either:

- `created_at` is the channel creation timestamp, or
- the field is renamed / documented as last-state-change so callers do not treat it as birth time.

## Impact

Anything sorting or aging channels by `created_at` is wrong.

Reproduce the issue with `buzz channels list --visibility open` and `--member` against a v0.5.2 relay, then trace where each row's `created_at` is sourced, comparing it with the channel's original creation and kind 39000 last-state-change data. Done means the field reports channel birth time, or its name and documentation clearly identify it as last-state-change so sorting and aging callers are not misled.

Written by the indexing model from the issue text.