docs(STATE-WP-0079): execute slice E2, retire 15 legacy interfaces

Retires the 15 workstream->workplan aliases that meet the retirement
standard: verified replacement, no manual hold, zero calls in a 7-day
window, and silence past the volume-scaled quiet threshold. Meter now reads
16 retired / 4 legacy with 0 outstanding candidates.

The 4 held are the three six-figure read paths and GET /tasks/?workstream_id
which still has traffic -- the set where a premature retirement would do the
most damage. They clear as their quiet counters run.

Journalled fleet-side in the-custodian/docs/retired-legacy-interfaces.md so
the record outlives this repo's archive.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

docs/retirement-cutover-slice-plan.md

@@ -242,7 +242,9 @@ the low-volume REST aliases, plus `PATCH /workstreams/{id}` (571 calls, quiet
 | `GET /workstreams/{id}` | 511 406 | quiet 19d of 60d |
 | `GET /workstreams/{id}/dependencies/` | 255 865 | quiet 42d of 60d |
 
-E2 can therefore proceed for the 15, which is the bulk of the slice. The four
+**Executed 2026-08-20:** the 15 are retired; the meter reads 16 retired / 4
+legacy with 0 outstanding candidates. Journalled at
+`the-custodian/docs/retired-legacy-interfaces.md`. The four
 held are the three six-figure read paths and the one with live traffic — exactly
 the set where a false retirement would hurt most. They clear on their own as the
 quiet counter runs, provided no new traffic arrives; the earliest,