chore: simplify logging output

[adrunkhuman]

Summary

• replace JSON/color formatter selection with one plain single-line stdout formatter
• include explicit UTC timestamps like 2026-05-21T20:30:00+00:00
• preserve contextual fields as simple key=value suffixes, with recursive sensitive-value redaction
• remove LOG_FORMAT docs and cover plain output, context fields, stdout, LOG_LEVEL, and timestamp behavior in tests

Verification

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

Review loop:

• fixed recursive redaction, mixed-key payload formatting, Python 3.13 taskName noise, and LOG_LEVEL env coverage
• final review signal was low with no confirmed actionable regressions
• docs and commenter passes completed

Closes #242