fix(review): round-2 findings — dead proxy key, unbounded transcribe, doc pins

- configure proxy: the bilibili_proxy key lost its only reader when yt-dlp
  exited the bilibili channel (PR5), leaving docs promising an unlock that
  did nothing. Now saves a generic `proxy` key (legacy key kept in sync),
  and every wording — CLI output, server tip, install.md quick reference —
  says what it really is: a saved address agents read to export
  HTTP(S)_PROXY, not an unlock switch
- transcribe: subprocess calls get timeouts (yt-dlp download 1800s, ffmpeg
  600s) so a stalled network read can't hang the CLI forever
- docs/update.md: rdt-cli install uses the same pinned git SHA as the
  code's _RDT_GIT_SOURCE instead of floating HEAD
- probe.py: documented as side-effect-free-probes-only (retries re-run
  verbatim with no backoff)

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

agent_reach/probe.py

@@ -53,6 +53,10 @@ def probe_command(
 ) -> ProbeResult:
     """Actually execute `cmd *args` and classify the result.
 
+    Intended for SIDE-EFFECT-FREE health probes only (version/status
+    commands): retries re-run the command verbatim with no backoff, so a
+    non-idempotent command would repeat its effect.
+
     package: pip/pipx package name used in the broken-install hint
              (defaults to cmd).
     """