chore: trim startup logs

[adrunkhuman]

Summary

• trim decorative and step-by-step startup log noise while keeping key lifecycle milestones
• add guild join/remove observability with setup state and permission checks
• include Send Messages in Threads in startup/join permission checks
• downgrade normal fixture announcement verification details to DEBUG

Verification

• uv run pytest
• uv run ruff check .
• uv run ruff format --check .
• uv run ty check typer_bot

Review loop:

• fixed missing thread-send permission coverage
• made guild join logging resilient to setup lookup failures
• tightened permission test fakes so newly checked permissions cannot pass accidentally
• final review signal was low with no confirmed actionable regressions
• docs and commenter passes completed

Closes #238