fix: prevent session token exfiltration via external app URLs (#26146) - #26303

Documentation Check

Updates Needed

• docs/user-guides/devcontainers/customizing-dev-containers.md - Removed the ### Session token section that documented $SESSION_TOKEN in devcontainer app URLs. This is correct: the pattern is now blocked for sub-agent apps.

• docs/user-guides/devcontainers/customizing-dev-containers.md - The new warning behavior is undocumented. When a sub-agent app URL contains $SESSION_TOKEN, the CLI now prints the raw URL and asks the user to substitute manually instead of auto-opening it. Users who relied on this pattern will encounter the warning with no docs to explain why or what to do next.

  Suggested addition in the Variable interpolation section (or as a note near the removed section's location):

  Note: $SESSION_TOKEN is not automatically substituted in devcontainer app URLs. Because devcontainer app URLs are registered at runtime by code running inside the workspace, Coder cannot verify their origin. If a devcontainer app URL contains $SESSION_TOKEN, the CLI prints the URL and asks you to substitute the token manually before opening it. To have the token substituted automatically, define the app in the workspace template instead of the devcontainer configuration.

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