refactor: always decrypt RX_LOG, remove rx_log_decrypt setting

[sg]

Summary

Removes the [bot] rx_log_decrypt setting. The client-side RX_LOG decrypt+ingest and the library's channel-log decryption are now always on.

Why

rx_log_decrypt only ever gated client-side decryption, but with it false:

• !path lost the routing path for channel messages (the path back-fill is correlated via decrypted RX_LOG logs), and
• the firehose hid decoded channel text.

The overhead of always decrypting is negligible (AES/HMAC per channel frame; one X25519 per DM frame), and the radio's private key is exported unconditionally anyway, so the only thing the toggle bought was a "capture-only / metadata-only" mode. For a command hub the priority is maximum usable data, so always-on wins: !path always works, channels beyond the radio's slot capacity are always covered, and the rx_log_decrypt↔!path footgun is gone.

Changes

• mcbot.py: drop the Config.rx_log_decrypt field + parse; set_decrypt_channel_logs(True) unconditionally; subscribe RX_LOG_DATA unconditionally; drop it from the startup banner; refresh comments. Queue/RX_LOG dedup unchanged.
• mcbot.conf.example: remove the rx_log_decrypt stanza; trim the stale note in the repeat_tracking docs.
• Bot_Usage.md: remove the config-table row and the capture-only sentence; reword to "always on."

Consequence

The firehose now always shows decoded RX_LOG channel text — the capture-only/metadata-only mode is intentionally removed.

Testing

py_compile clean; Config no longer exposes rx_log_decrypt; all five test suites (logging, repeat-tracking, eviction, decode-advert, advert-interval) pass; no references remain repo-wide.