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

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