fix(415): keep archived api.channel-health-signal rationale verbatim

The supersession appended a 'Why superseded' note to the archived #72 record, which
the diff-mode decisions validator (CI 'decisions lifecycle' job) flags as a rationale-prose
change requiring [decisions-edit]. The archive's own rule is rationale-kept-verbatim, and the
'why' is already carried by the superseded-by link, the Mechanics forwarding pointer, the
archive header, and the successor record api.channel-health-object. Drop the note.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

docs/decisions/archive/api.md

@@ -54,10 +54,3 @@ Keeping all three out held #72 to a **read-path-only** change: no migration, no
 channel editor's playout-source guard. Both call sites now share `Mapper.GetPlayoutsCount` (Mirror-aware).
 The related *silent* server-side coercion of Mirror→Generated (a 200 that discards the caller's intent,
 against the §3 "surface it, don't silently filter" rule) is filed as **#401**, not fixed here.
-
-**Why superseded (#415, 2026-07-23):** the auto-tune status taxonomy churn (#383/#384) this record was
-waiting on has landed (see `channel.origin-marker`, #414), clearing the stated blocker. `api.channel-health-object`
-replaces the raw `PlayoutCount`-as-verdict with a server-derived `health` object computed from the
-built timeline, and computes both previously-deferred fault classes (empty schedule, broken source)
-kind-agnostically across all 5 `PlayoutScheduleKind` values — closing the #71 trap this record's own
-`EmptyScheduleHealthCheck` reference fell into. `PlayoutCount` itself is retained on the DTO unchanged.