Add Slack agent wrapper with command and rich Block Kit support

[phenobarbital]

Motivation

• Add Slack as a first-class integration so AI-Parrot agents can be exposed via Slack Events API and slash command webhooks.
• Provide a simple built-in command surface (help, clear, commands) and per-channel/user conversation memory for interactive Slack usage.
• Render agent output as Slack Block Kit content (markdown sections, code blocks, tables, images/context) for richer responses.

Description

• Add parrot/integrations/slack/wrapper.py implementing SlackAgentWrapper which handles Events API webhooks, slash commands, per-session memory, agent calls (agent.ask) and builds Slack Block Kit payloads for text, code, tables and images.
• Add parrot/integrations/slack/models.py defining SlackAgentConfig with env fallback for bot_token and signing_secret, and parrot/integrations/slack/__init__.py to export the Slack integration.
• Wire Slack into the unified integration model by updating parrot/integrations/models.py to recognize kind: slack and validate required credentials.
• Update parrot/integrations/manager.py to initialize and track SlackAgentWrapper instances during startup and include them in shutdown cleanup.
• Add unit tests tests/test_slack_integration.py that validate block generation and IntegrationBotConfig parsing for Slack.

Testing

• Static verification: ran python -m py_compile parrot/integrations/slack/models.py parrot/integrations/slack/wrapper.py parrot/integrations/slack/__init__.py parrot/integrations/models.py parrot/integrations/manager.py parrot/integrations/__init__.py tests/test_slack_integration.py and it succeeded.
• Attempted test run with pytest -q tests/test_slack_integration.py tests/test_integration_wrappers.py but test collection failed due to missing optional runtime dependencies (ModuleNotFoundError: markdown2, ModuleNotFoundError: aiohttp) in the environment.
• Tried to install the missing packages with pip install aiohttp markdown2 -q but network/proxy restrictions prevented installation in this environment, so full pytest run could not be completed here.

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Codex Task