config: support toml and yaml flavor configs

[PerishCode]

Why

flavor only accepted JSON configs, while the surrounding repo infrastructure is
moving toward TOML-first config files. The repo self-check config should follow
that shape before peta :cli doctor starts treating it as baseline.

What

• Add JSON, TOML, YAML, and YML config parsing for explicit --config paths.
• Extend walk-up discovery to flavor.toml, flavor.yaml, flavor.yml, and
  flavor.json, with TOML-first priority.
• Move this repo's self-check config from flavor.json to flavor.toml.
• Update guard, release, smoke, init, README, and AGENTS references to use
  flavor.toml.
• Bump flavor-cli to 0.3.2 for the stable release.

Tests

• cargo fmt --all --check
• cargo clippy --locked --workspace --all-targets -- -D warnings
• cargo test --locked --workspace
• cargo run --locked -p flavor-cli -- check --root . --config flavor.toml
• python3 scripts/init.py
• commit hook: fmt, cargo check, flavor self-check, shell syntax, Python syntax;
  PowerShell syntax skipped locally because pwsh is not installed.

Compatibility

flavor.json remains supported, but discovered configs now prefer
flavor.toml, then flavor.yaml, then flavor.yml, then flavor.json when
multiple supported files are present in the same directory.