Emit a /.well-known/matrix/mautrix file listing the base URLs of all
enabled and exposed mautrix bridges under the `fi.mau.bridges` property,
so tools like Mautrix Manager (https://github.com/mautrix/manager) can
auto-discover them.

The list is built in group_vars from each bridge's public address and is
gated on the bridge being enabled, the playbook attaching its Traefik
labels, and the exposure router being emitted, so we only advertise URLs
that are actually reachable. The file follows the same auto/custom and
configuration-extension pattern as the other well-known files and is only
written when the list is non-empty.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

The companion role was tightly coupled to Synapse through shared tags, worker routing, and lifecycle ordering. Keeping them separate added coordination overhead without practical benefits, especially for parallelized execution.

This merges the role into matrix-synapse while keeping companion logic organized under dedicated reverse_proxy_companion task/template subdirectories.

Compatibility is preserved:
- matrix_synapse_reverse_proxy_companion_* variable names remain unchanged
- install/setup companion-specific tags remain available

Cross-role/global wiring is now in group_vars (matrix-synapse section), while role defaults provide sensible standalone defaults and self-wiring for Synapse-owned values.