chore: remove stale requirements.txt in favor of pyproject.toml (#13515)

The root requirements.txt has drifted from pyproject.toml for years
(unpinned, missing deps like slack-bolt, slack-sdk, exa-py, anthropic)
and no part of the codebase (CI, Dockerfiles, scripts, docs) consumes
it. It exists only for drive-by 'pip install -r requirements.txt'
users and will drift again within weeks of any sync.

Canonical install remains:
    pip install -e ".[all]"

Closes #13488 (thanks @hobostay — your sync was correct, we're just
deleting the drift trap instead of patching it).

requirements.txt

@@ -1,36 +0,0 @@
-# NOTE: This file is maintained for convenience only.
-# The canonical dependency list is in pyproject.toml.
-# Preferred install: pip install -e ".[all]"
-
-# Core dependencies
-openai
-python-dotenv
-fire
-httpx
-rich
-tenacity
-prompt_toolkit
-pyyaml
-requests
-jinja2
-pydantic>=2.0
-PyJWT[crypto]
-debugpy
-
-# Web tools
-firecrawl-py
-parallel-web>=0.4.2
-
-# Image generation
-fal-client
-
-# Text-to-speech (Edge TTS is free, no API key needed)
-edge-tts
-
-# Optional: For cron expression parsing (cronjob scheduling)
-croniter
-
-# Optional: For messaging platform integrations (gateway)
-python-telegram-bot[webhooks]>=22.6
-discord.py>=2.0
-aiohttp>=3.9.0