fix(cli): report channels list last-write as updated_at, keep created_at as deprecated alias

[sanjay3290]

Fixes #6390.

Problem

buzz channels list (and channels get) reports the kind 39000 channel-metadata event's created_at as the channel's created_at. Kind 39000 is addressable/replaceable, so that timestamp is overwritten on every metadata update. The field therefore reports the last metadata write, not the channel's creation. The channel's birth time is not recoverable from a replaceable event, so it cannot be fixed by finding a better source.

Change

Name the field honestly instead of guessing a birth time:

• Add updated_at, carrying the same value under an accurate name.
• Keep created_at as a deprecated alias of updated_at for one release so existing scripts do not break silently.
• Document the semantics on extract_channel_metadata and in buzz channels list --help.

Both call sites (channels list at channels.rs:103, channels get at :248) go through extract_channel_metadata, so the change lands in one place by construction.

Removing created_at outright is out of scope for this PR — that is the follow-up once the deprecation window closes.

Tests

Three tests were written first and watched fail, then the fix was applied (two commits, tests before fix):

• updated_at_carries_kind_39000_created_at
• created_at_is_deprecated_alias_of_updated_at
• missing_created_at_yields_zero_for_both_fields

All three call the real extract_channel_metadata the binary uses.

Deletion probes (both keep the code compiling and produce real failures):

• Remove the updated_at line → 3 failed.
• Remove the created_at alias line → 2 failed.

Gates

At f7ac2b5d0, clean tree:

• ./scripts/run-tests.sh unit (the just test-unit path when cargo-nextest is absent) → all 9 packages pass, 107s.
• cargo clippy --workspace --all-targets -- -D warnings → clean.
• cargo fmt --all -- --check → clean.

just test-integration / just ci need Docker and were not run.