asahi: HM takeover of existing /home/da — backups, stale hm-v3 state, substituter shadowing

[cernoh]

ASAHI's da home was previously managed by standalone hm-v3 (home-manager switch as user service). After the dendritic switch, HM activation moves in-tree via home-manager-da.service. Expect and remediate:

• Dotfile churn: home-manager.backupFileExtension = "hm-backup" will rename any pre-existing file the new config also manages. Sweep after first switch:

  find /home/da -name '*.hm-backup' -not -path '*/.cache/*'

  Diff each against its replacement; delete or port customizations back through this repo.
• Old hm-v3 generations linger in da's user profile (home-manager generations). They still activate on demand and can confuse rollbacks. Once the new path is stable:

  home-manager expire-generations "1 day ago"

  (run as da; do NOT run the old hm-v3 checkout's switch again afterwards)
• User-level nix conf shadowing: hm-v3 may have left/generated ~/.config/nix/nix.conf. A stale one overrides system substituters/trusted settings for that user (documented failure mode: cache.nixos.org silently dropped → massive source builds). Check content after switch; if it exists, either empty it or let the flake's HM layer own it.
• Session env deltas: EDITOR/VISUAL now pinned to nvim via programming module; zellij default_shell=fish; tmux direnv update-environment list changed. Nothing to fix — just re-login rather than reusing pre-switch shells when verifying.

Related: #63 (pre-switch gate), #62.

[cernoh]

Status from running these on the Mac today:

User-level nix.conf shadowing: CLEAN. ~/.config/nix does not exist for da — nothing can shadow the system substituters post-switch. The flake also carries its own nixConfig, so consumers accept caches without a user conf. Nothing to do here.

Session env deltas: acknowledged; verification after switch will happen from fresh logins only.

Dotfile churn (*.hm-backup) and stale hm-v3 generations: both are post-first-switch sweeps — they can only be done after sudo nixos-rebuild switch --flake ~/.config/dendritic#ASAHI runs. Checklists stay open until then. Current pre-switch state: hm-v3 generation 145 is active (home-manager generations), no .hm-backup files exist yet.

[cernoh]

No new executable steps from NIXPC: the remaining items (*.hm-backup sweep, hm-v3 generation expiry as da, re-login verification) all run after the first sudo nixos-rebuild switch on the Mac, which has not happened yet (tracked in #62). Predicate check still holds: ~/.config/nix absent for da, nothing can shadow system substituters.

[cernoh]

Agent pass (2026-08-28, from ASAHI):

• Dotfile churn: no *.hm-backup files under /home/da today (pre-switch for the current train; the sweep + diff runs after the next nixos-rebuild switch, which besides sudo also can't be approximated from this session).
• ~/.config/nix: absent — nothing shadows the flake substituters (checked; the known cache.nixos.org drop-out failure mode cannot apply).
• expire-generations: deferred — the new in-tree HM path (home-manager-da.service) only activates on the switch; running it now would touch the old hm-v3 profile prematurely. Item remains pending on the switch.
• Session env deltas: EDITOR/VISUAL=nvim, zellij default_shell=fish, tmux update-environment list are all wired config-side (attrs/programming + fish/nushell checkout symlinks, PR feat(shells): deliver fish/nushell config as live-editable checkout symlinks #105); they materialize on the switch + fresh login.

Note: with #95 (PR #102) EDITOR/VISUAL move to environment.sessionVariables (system-side) rather than home.sessionVariables.