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>

AGENTS.md

@@ -36,6 +36,10 @@ git-tracked in `agent/`.
 - **Conventions** (`agent/CONVENTIONS.md`): in-flake state, menu
   placement, Waybar parity with whole-swaps, toggle-vs-package discipline,
   template as SoT for opt-in comments (`templates/downstream`).
+- **Sync sweep on completion** (`agent/LOOP.md` §5): when a task ships,
+  update or delete every cross-reference it made stale — BACKLOG
+  pitches/pointers, HARDWARE-QUEUE entries, docs — in the same commit.
+  No stale items, no unsynced information.
 - **Git:** direct commits on `main`, pushed; **`v1` is human-only** —
   never touch it. Never `nix flake update` unless the task is a lock bump.
   No formatter — match aligned hand-formatting.