Backport upstream v1.18 security hardening (debug config redaction, http(s)-only browser URLs, escaped OAuth error page) #1506

Three hardening fixes upstream shipped in v1.18 that our fork (based on v1.17.9) does not have yet:

1. debug config prints credentials. altimate-code debug config writes the resolved configuration with provider API keys, tokens and auth headers in clear text. Users paste this output into bug reports.
2. Browser opens any URL scheme. Every browser-opening call passes the URL straight to open(). The most exposed path is MCP OAuth: the authorization URL comes from the MCP server, so a malicious or compromised server can make us open file:// or a custom-scheme URL, which launches local apps. Model-written markdown links in the TUI are also opened unchecked.
3. Unescaped text on the Snowflake OAuth error page. The local callback page interpolates error_description from the callback query into HTML without escaping.

Upstream fixes: 82d4c89031, 29f07e0c69, and the escaping part of e8fea9e63a.