[codex] add L5-T3 realtime task card

[Ridealist]

Summary

• Add the L5-T3 realtime practice task card and register it in the task-card manifest.
• Switch the tracked markdown default practice task card to special_activity_plan.
• Update realtime/admin fallbacks and tests for the new default card.
• Make opening extraction stop at the first paragraph so placeholder content does not leak into the opening sentence.

Validation

• pnpm prompts:check
• UV_CACHE_DIR=/tmp/uv-cache uv run python -m pytest agent/tests/test_prompt_realtime.py agent/tests/test_realtime_prompt_source.py -q
• pnpm --dir client exec node --test tests/realtime-prompt-route.test.mjs tests/realtime-prompt-config.test.mjs
• Earlier full checks after the opening update: UV_CACHE_DIR=/tmp/uv-cache uv run python -m pytest agent/tests -q, pnpm --dir client test

Notes

• This is a stacked draft PR based on codex/admin-prompt-version-labels to keep the diff limited to the L5-T3 task-card work.