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

4 files changed, 38 insertions, 19 deletions

diff --git a/etc/pacman.conf b/etc/pacman.conf
index a2cba6c..7097940 100644
--- a/etc/pacman.conf
+++ b/etc/pacman.conf
@@ -27,12 +27,6 @@ Architecture = auto
 IgnorePkg   = llama.cpp-vulkan
 #IgnoreGroup =
 
-# external-editor-revived (AUR) hard-depends on `thunderbird`, but we run
-# Thunderbird as the org.mozilla.thunderbird flatpak instead. Tell pacman to
-# pretend the system thunderbird package is installed so the dep is satisfied
-# without pulling in the binary. Version is arbitrary — only the name matches.
-AssumeInstalled = thunderbird=999.0-1
-
 #NoUpgrade   =
 #NoExtract   =
 
diff --git a/meta/base.txt b/meta/base.txt
index a226674..f77b85b 100644
--- a/meta/base.txt
+++ b/meta/base.txt
@@ -195,12 +195,10 @@ protonmail-bridge-core
 perl-authen-sasl
 perl-mime-tools
 perl-net-smtp-ssl
-# Native messaging host binary for External Editor Revived; bridged into
-# the TB flatpak by run_onchange_after_deploy-tb-eer.sh.tmpl.
-# Pulls `thunderbird` as a hard dep, which we don't want (we use the
-# org.mozilla.thunderbird flatpak); pacman.conf's AssumeInstalled rule
-# pretends thunderbird is installed so this resolves without the package.
-external-editor-revived
+# (External Editor Revived's native-messaging host is installed via nix
+# on the host — see nix/host.nix — so we don't pay the AUR variant's
+# hard `thunderbird` dependency. The bridge into the TB flatpak is
+# wired up by run_onchange_after_deploy-tb-eer.sh.tmpl.)
 
 # --- media (mpv is the io.mpv.Mpv flatpak in meta/flatpak.txt; streamlink
 # and yt-dlp run on the host and pipe/launch into the flatpak mpv via
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 $EDITOR. We run TB as the
+  # org.mozilla.thunderbird flatpak; the AUR package would drag in system
+  # `thunderbird` as a hard dep, so we take it from nixpkgs here instead
+  # (the nix derivation has no mailer dep). The bridge wiring lives in
+  # run_onchange_after_deploy-tb-eer.sh.tmpl; it auto-detects the binary
+  # under ~/.nix-profile and the manifest gets relocated into the TB
+  # flatpak sandbox.
+  home.packages = with pkgs; [
+    external-editor-revived
+  ];
+
   # ── Smartcard (Yubikey) ────────────────────────────────────────────────────
   # Nix's gnupg ships its own scdaemon. Delegate to the system pcscd
   # service instead of letting nix's scdaemon open the USB device
diff --git a/run_onchange_after_deploy-tb-eer.sh.tmpl b/run_onchange_after_deploy-tb-eer.sh.tmpl
index 6a7690e..27f1fef 100644
--- a/run_onchange_after_deploy-tb-eer.sh.tmpl
+++ b/run_onchange_after_deploy-tb-eer.sh.tmpl
@@ -8,20 +8,34 @@
 # Idempotent. Re-runs on script changes or whenever the host-side manifest
 # content changes.
 #
-# host manifest hash: {{ output "sh" "-c" "for p in /usr/lib/mozilla/native-messaging-hosts/external_editor_revived.json /usr/lib/thunderbird/native-messaging-hosts/external_editor_revived.json; do [ -f \"$p\" ] && sha256sum \"$p\" && break; done; true" | sha256sum }}
+# host manifest hash: {{ output "sh" "-c" "for p in $HOME/.nix-profile/lib/mozilla/native-messaging-hosts/external_editor_revived.json /usr/lib/mozilla/native-messaging-hosts/external_editor_revived.json /usr/lib/thunderbird/native-messaging-hosts/external_editor_revived.json; do [ -f \"$p\" ] && sha256sum \"$p\" && break; done; true" | sha256sum }}
 set -eu
 
 TB_APP=org.mozilla.thunderbird
 MANIFEST_NAME=external_editor_revived.json
 WRAPPER_NAME=external_editor_revived.sh
-HOST_BINARY=/usr/lib/external-editor-revived/external-editor-revived
+
+# Locate the EER binary. nixpkgs is the preferred source (see nix/host.nix)
+# because it has no thunderbird dep; the AUR package is the fallback for
+# hosts where nix isn't set up yet.
+HOST_BINARY=
+for candidate in \
+  "$HOME/.nix-profile/bin/external-editor-revived" \
+  /usr/lib/external-editor-revived/external-editor-revived \
+  /usr/bin/external-editor-revived; do
+  if [ -x "$candidate" ]; then
+    HOST_BINARY=$candidate
+    break
+  fi
+done
 
 flatpak info --user "$TB_APP" >/dev/null 2>&1 || exit 0
 
-# Locate the host-side manifest. Different PKGBUILDs have used different
-# install dirs; prefer the canonical mozilla path, fall back to TB-specific.
+# Locate the host-side manifest. Different packagings (nix, AUR variants)
+# use different install dirs; check the common ones.
 HOST_MANIFEST=
 for candidate in \
+  "$HOME/.nix-profile/lib/mozilla/native-messaging-hosts/$MANIFEST_NAME" \
   /usr/lib/mozilla/native-messaging-hosts/$MANIFEST_NAME \
   /usr/lib/thunderbird/native-messaging-hosts/$MANIFEST_NAME \
   /usr/lib64/mozilla/native-messaging-hosts/$MANIFEST_NAME; do
@@ -31,12 +45,12 @@ for candidate in \
   fi
 done
 [ -n "$HOST_MANIFEST" ] || {
-  echo "tb-eer: external-editor-revived manifest not found on host; install the package or skip TB native editor." >&2
+  echo "tb-eer: external-editor-revived manifest not found on host; install via nix (host.nix) or skip TB native editor." >&2
   exit 0
 }
 
-[ -x "$HOST_BINARY" ] || {
-  echo "tb-eer: $HOST_BINARY not executable on host; skipping." >&2
+[ -n "$HOST_BINARY" ] || {
+  echo "tb-eer: external-editor-revived binary not found on host; install via nix (host.nix) or skip TB native editor." >&2
   exit 0
 }