chore(agent): sync-sweep rule on task completion + apply it; zram user docs

New standing rule (LOOP.md §5 step 4, echoed in AGENTS.md): when a task
ships, grep its number/feature name across agent/ and docs/ and update
or delete every cross-reference it made stale, in the same commit —
including pruning checked-off [x] HARDWARE-QUEUE entries whose outcome
is recorded (journal/ROADMAP; git history is the archive).

First application: BACKLOG PROPOSED groomed (the auto time-of-day theme
pitch was stale — shipped as #79; shipped/promoted placeholder lines
removed; v1.0 pointer rewritten; FUTURE promoted-stub dropped);
HARDWARE-QUEUE pruned of 29 checked entries (363→253 lines) with
dangling references reworded and the prune rule documented in its
header.

Also closes the zram user-doc gap: README §3 now documents the zram
default (zstd/50%/prio 100), hibernation working out of the box on new
installs, and the MIGRATION.md runbook for existing machines.

Verification: V0 (docs/meta only).

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

agent/LOOP.md

@@ -132,6 +132,14 @@ the reusable recipes).
    file's rules).
 2. Append a `JOURNAL.md` entry (template in that file).
 3. Queue any V3 checks in `HARDWARE-QUEUE.md`.
+4. **Sync sweep.** Grep the item's number and feature name across
+   `agent/` and `docs/` and update or delete every cross-reference the
+   ship made stale: the PROPOSED pitch that spawned it (and BACKLOG's
+   v1.0 pointer), HARDWARE-QUEUE entries it supersedes, ROADMAP/README/
+   docs mentions of the old behavior. While there, prune checked-off
+   `[x]` HARDWARE-QUEUE entries whose outcome is already recorded
+   (journal/ROADMAP — git history is the archive). A shipped item must
+   leave no stale pointer behind; the sweep rides in the same commit.
 
 ### 6. Pace (self-paced runners only)
 Under `/loop`, continue to the next iteration while tasks remain