Since mautrix-meta v26.07, Instagram is a separate bridge upstream (its own binary, container image and configuration schema). As of v0.2608.0, the bridge no longer has a `mode` configuration option at all, so the premise of auto-generating the Instagram role from the Messenger one (one binary, one schema, a mode switch) is gone.

- Drop the role generator (`bin/rebuild-mautrix-meta-instagram.sh` and the `just rebuild-mautrix-meta-instagram` recipe). The two roles are now maintained independently.

- Drop `matrix_bridge_mautrix_meta_instagram_meta_mode` and all mode-derived lookups from the Instagram role. Computed values are unchanged.

- Sync both config templates with the v0.2608.0 example configs: the Instagram template loses `mode` and gains the Instagram-only options; the Messenger template replaces `mode` with the new `tor` option (still derived from `matrix_bridge_mautrix_meta_messenger_meta_mode: facebook-tor`) and loses the Instagram-only `disable_xma_*` options.

- Fix self-building for the Instagram role: build via `Dockerfile.ig` (producing the actual Instagram binary at `/usr/bin/mautrix-instagram`) instead of the Messenger `Dockerfile`. A new `matrix_bridge_mautrix_meta_instagram_container_binary_path` variable points the systemd service at the right binary for self-built and prebuilt images.

- Rename the systemd service templates to role-specific file names.

Related to https://github.com/spantaleev/matrix-docker-ansible-deploy/issues/5510

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>