Add Discord agent gateway

[michaelmwu]

Description

Adds the first Discord agent gateway path: a thin /agent cog forwards natural-language task requests to backend API endpoints that parse, policy-check, audit, confirm, and execute supported inline task actions.
The shared five08.agent package defines request/plan/result models, deterministic tool manifests, capability policy, an inline task tool registry, model-tier routing with fallback support for OpenAI-compatible providers, and the initial task-command orchestrator.
Docs now cover the bot operating model, agent gateway flow, model-tier configuration, current pending-plan storage limits, and AGENT_API_TIMEOUT_SECONDS configuration.

Related Issue

Fixes #259
Related to #257
Follow-up context/memory design: #265

How Has This Been Tested?

• ./scripts/format.sh
• ./scripts/lint.sh
• ./scripts/mypy.sh
• uv run pytest tests/unit/test_agent_gateway.py tests/unit/test_backend_api.py tests/unit/test_worker_config.py -q
• uv run pytest tests/unit/test_agent_gateway.py tests/unit/test_backend_api.py tests/unit/test_agent_cog.py -q
• ./scripts/test.sh

Summary by CodeRabbit

• New Features

  • /agent Discord command with interactive confirm/cancel flow, backend request + confirmation endpoints, synchronous gateway and single-use confirmation plans.
  • Tiered agent model selection (fast/strong/reasoning) with provider fallbacks, agent API timeout, and in-process task tooling for create/update/search actions.
  • Optional LLM tracing/observability via LANGFUSE base URL.

• Documentation

  • Updated README/ENVIRONMENT with agent gateway, new env vars, timeout, fallback order, and updated default models to gpt-5-mini.

• Tests

  • Expanded unit tests covering bot, gateway, orchestrator, tools, policy, and backend confirmation flows.