feat: add Maestro Discord agent rollout, templates, and setup docs

[Vardominator]

Summary

• add a secure multi-instance Discord rollout pattern for functional agents using Maestro as an example
• add agent templates for SOUL.md and TOOLS.md:
  • brain-qa, github-manager, notion-manager, infra-triage, deep-research
• add data source convention/template for knowledge-style agents (DATA_SOURCES.md)
• add docs for bot access setup, integrations/use-cases, and Discord E2E rollout
• add inventory entries for the 3 Discord instance groups (knowledge, systems, infra)
• wire additional env passthrough for tool-backed agents in compose generation:
  • GITHUB_TOKEN, NOTION_API_KEY, BETTERSTACK_API_TOKEN, BETTERSTACK_API_BASE_URL

Security / Sensitivity Check

• no real tokens, secrets, or private keys are committed in this PR
• Maestro-specific sensitive identifiers were scrubbed from examples:
  • removed hard-coded Discord guild/channel IDs from instances/maestro-discord-systems/config/instance.overrides.example.json5
  • replaced with explicit REPLACE_WITH_* placeholders
• all credentials remain environment-based placeholders in docs/config examples

Validation

• bun test passes locally (24 passed, 0 failed)
• validated compose env passthrough behavior with updated unit tests in tests/compose.test.ts

Notes

• Maestro naming is intentionally kept as an example rollout pattern.
• operational/runtime state files remain gitignored and are not part of this PR.