feat: discover default .dot.toml configuration

[yslib]

Summary

• make .dot.toml the implicit manifest name and discover it from the current directory, then the platform-specific user configuration directory
• add a typed ConfigRequest and resolve the manifest once before command dispatch
• preserve explicit --config, deterministic no-fallback-on-read-error behavior, dangling-symlink selection, and isolation from injected platform data
• update the CLI help and documentation, with real-binary and cross-platform test coverage

Why

dot should be usable from any directory without repeatedly passing --config, while retaining conservative and predictable configuration discovery.

User impact

• Explicit --config continues to accept any file name and bypasses discovery.
• Without --config, dot first checks ./.dot.toml.
• Linux and macOS then fall back to ~/.config/dot/.dot.toml.
• Windows falls back to %APPDATA%\dot\.dot.toml.
• The legacy implicit ./dot.toml name is no longer discovered.

Validation

• cargo fmt --all -- --check
• cargo clippy --locked --all-targets --all-features -- -D warnings
• cargo test --locked --all-targets --all-features (290 top-level tests, 0 failures)
• strict Clippy for x86_64-pc-windows-gnu
• isolated explicit/local/fake-home/missing-config smoke checks
• git diff --check main...HEAD