refactor(eer): install external-editor-revived via nix on the host

The AUR `external-editor-revived` PKGBUILD declares a hard `thunderbird`
dependency, which blocks removing the unused system Thunderbird binary
alongside the org.mozilla.thunderbird flatpak (and pacman's
`AssumeInstalled` is a CLI flag, not a pacman.conf directive, so the
previous workaround was nonfunctional).

Nixpkgs' `external-editor-revived` is just `rustPlatform.buildRustPackage`
plus a relocatable native-messaging manifest — zero mailer dep — so the
host gets it from nix instead.

* nix/host.nix: add `external-editor-revived` to `home.packages`. Kept
  out of `common.nix` so the remote-dev VM (which has no Thunderbird)
  doesn't carry the build closure.
* run_onchange_after_deploy-tb-eer.sh.tmpl: search
  `~/.nix-profile/{bin,lib/mozilla/native-messaging-hosts}` first and
  fall through to the legacy pacman paths. The chezmoi manifest-hash
  probe now checks the nix path too, so the hook re-runs cleanly when
  nix bumps the EER version.
* meta/base.txt: drop the `external-editor-revived` AUR entry and
  rewrite the comment to point at the nix declaration.
* etc/pacman.conf: revert the bogus `AssumeInstalled` directive
  (CLI-only, not pacman.conf).

On-host migration:

    home-manager switch --flake ~/dotfiles/nix#host    # picks up EER
    sudo pacman -Rns external-editor-revived thunderbird mpv
    chezmoi apply -v                                   # re-runs tb-eer hook

1 files changed, 13 insertions, 0 deletions

diff --git a/nix/host.nix b/nix/host.nix
index eac2a20..96adfba 100644
--- a/nix/host.nix
+++ b/nix/host.nix
@@ -13,6 +13,19 @@
   home.username = builtins.getEnv "USER";
   home.homeDirectory = builtins.getEnv "HOME";
 
+  # ── Thunderbird helpers (host only) ────────────────────────────────────────
+  # external-editor-revived is the native-messaging host that lets the
+  # Thunderbird addon hand a composing draft to $EDITOR. We run TB as the
+  # org.mozilla.thunderbird flatpak; the AUR package would drag in system
+  # `thunderbird` as a hard dep, so we take it from nixpkgs here instead
+  # (the nix derivation has no mailer dep). The bridge wiring lives in
+  # run_onchange_after_deploy-tb-eer.sh.tmpl; it auto-detects the binary
+  # under ~/.nix-profile and the manifest gets relocated into the TB
+  # flatpak sandbox.
+  home.packages = with pkgs; [
+    external-editor-revived
+  ];
+
   # ── Smartcard (Yubikey) ────────────────────────────────────────────────────
   # Nix's gnupg ships its own scdaemon. Delegate to the system pcscd
   # service instead of letting nix's scdaemon open the USB device