artifacts-gc: cursor-continue sweep + older legacy name schemes

Status summary#

Follow-up to #2565, prompted by the first real backfill pass (2026-09-02).

Quadratic re-walking + inflated counters. The sweep restarts its listing from the head after every page that deleted anything (the original cursor-invalidation paranoia). Skipped repos compact toward the head of the oldest-first listing as orphans around them are deleted, so each restart re-walks — and re-counts — the whole skipped prefix. preview_3 reported skippedForeign: 276994 in a 5000-delete run whose namespace holds nowhere near that; the real foreign count is the ~5–10k the restarts multiplied. ~28% of the run's API calls were redundant list pages, from a shared 1200-req/5-min account budget, and the overhead grows as the deletable share shrinks.
Two older name schemes are unrecognized (so never reclaimed):
- repo-<hex> — hex of <id>:<path>, e.g. decodes to __null__:/repos/iterate-config-base (~2.3k on preview_2 alone).
- proj__os__<ulid>--<suffix> — the pre-prj_<hex> project id format; liveness-checkable against the directory KV exactly like current ids.

Always continue the cursor, never restart. We only delete entries at or before the cursor's position in the created_at-ascending order, so forward iteration is unaffected; worst case a stale cursor skips entries, and the next run's fresh listing self-heals (same story as failed deletes). Counters become walk-once accurate.
Extend triage: recognize proj__<app>__<id> leading ids (KV liveness check as usual) and decode repo-<hex> (id __null__ → global-scope handling; other ids → KV liveness check). Anything else stays untouched.

cursor-continue sweep (no restart-from-head; drop the now-dead branch) — sweep loop in artifacts-gc.ts; cursor-safety argument in the comment
triage: proj__*__ ids — parseLegacyRepoName
triage: repo-<hex> decode — parseLegacyRepoName, printable+separator fence, garbage decodes stay foreign
tests for the new shapes; existing tests stay green — 5 pass
live validation on a real namespace after the running drain pass completes (avoid contending with it)

(added as work proceeds)