feat: the Discord bot token can live in the secrets proxy

[praxagent]

The Discord bot token was the one credential that couldn't move to the secrets proxy. Proxy side: praxagent/prax-secrets-proxy#8.

What was missed in July

The 2026-07-22 verdict (LOCAL: a 401, "no header to inject") came from header-only injection. The MITM forward proxy can do both remaining parts:

• REST: inject Authorization: Bot <token> using a header prefix.
• Gateway: rewrite the IDENTIFY and RESUME messages the client sends, setting d.token. discord.py sends them as plain JSON text: the gateway uses compress=0, and zlib-stream compresses only the server's side.

Changes

• credential_registry: DISCORD_BOT_TOKEN is now PROXY_FORWARD, inject="discord-bot", exclusive=True.
  • The exported forward map includes Discord only when its caller is named: --exclusive-callers DISCORD_BOT_TOKEN=prax-prod.
  • Otherwise it is skipped, with the reason given. A dev Prax holding the placeholder can never become the bot too (Rule 0).
• Tests: the registry tests cover exported-only-with-callers, the two rules, the prefix, and that only exclusive credentials carry callers.
• Docs:
  • secrets-proxy.md gets a section with the rollout (the proxy holds the token, its own prax-prod caller, regenerate the map, Prax gets a placeholder plus DISCORD_USE_PROXY=true, then rotate the token);
  • credentials.md and deployment-topology.md are updated;
  • the VERIFICATION_LEDGER row is updated.

Verified

• Live, with real discord.py 2.7.1 through the real mitmproxy against a fake Discord:
  • the named caller logged in, and its IDENTIFY carried the injected token;
  • another caller got 401 and LoginFailure.
  • Not run against Discord itself.
• make ci green: 4013 passed. No secrets-proxy requests during the run beyond my own .invalid probe.

Rollout

This needs the proxy's #5, #6 and #8 merged and deployed, and production .env changes, which are yours to make. Steps are in the doc.