feat(suspend): re-enable suspend on s2idle, drop diagnostic scaffolding

Confirmed root cause: this hardware's S3 (deep) firmware path triggers a
fatal wake-from-suspend hang only on linux-hardened. INIT_ON_FREE + slab
hardening + tighter locking turn a latent driver race that stock linux
gets away with into an unrecoverable panic so early the journal isn't
even flushed. mem_sleep_default=s2idle bypasses the BIOS S3 path
entirely (s0ix is a pure-kernel low-power state) and suspends/resumes
reliably under hardened.

This is a widespread Lenovo S3 firmware issue across post-2018
ThinkPads (see Ubuntu T560, X1C9/10/11 reports). Lenovo themselves
moved newer firmwares to s2idle-only. Not a linux-hardened bug per se;
just hardened being a strict enough kernel to make the bug fatal.

Keep:
* mem_sleep_default=s2idle in etc/kernel/cmdline-linux-hardened.tmpl
  (only the hardened UKI; stock linux keeps unchanged shared cmdline)

Revert (all the diagnostic / speculative scaffolding from the last
few commits):
* MODULES=(intel_lpss_pci) → MODULES=()  — Arch wiki touchpad fix was
  not the cause here
* nmi_watchdog=panic softlockup_panic=1 panic=10 — only needed to
  auto-reboot during diagnosis
* no_console_suspend — diagnostic-only
* etc/systemd/logind.conf.d/20-no-suspend.conf  — masking workaround
* sleep-target masking block in run_onchange_after_deploy-etc.sh.tmpl,
  replaced with a one-shot cleanup that removes any leftover
  /dev/null symlinks from systems that ran the previous version
* systemd-pstore.service from systemd-units/system.txt — added only to
  catch the diagnostic panic
* diagnose-suspend.sh helper (and its .gitignore/.chezmoiignore entries)
* sway suspend → lock-session keybind workaround
* power-menu.sh Suspend entry restoration
* KEYBINDS.md docs

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