Layout-aware conversion direction, explicit API, and agent workflow docs

[qqrm]

Motivation

• Punctuation-only or letter-absent inputs could be converted in the wrong direction because the old heuristics had no explicit direction API.
• Prefer the user-visible/foreground keyboard layout when letter counts are tied or absent so conversions match user expectations.
• Ensure the contributor/agent workflow mandates running and addressing formatting, linting, build, and test checks before finalizing changes.

Description

• Introduced ConversionDirection and convert_ru_en_with_direction and added conversion_direction_for_text plus letter-count helpers in src/domain/text/mapping.rs to allow explicit-direction conversions and deterministic behavior for punctuation-only inputs.
• Implemented expected_direction_for_foreground_window() and layout helpers in src/domain/text/convert.rs to derive a layout-based fallback from the HKL/LANGID of the foreground window.
• Updated selection- and last-word conversion flows to prefer conversion_direction_for_text and fall back to the foreground layout via a convert_with_layout_fallback wrapper, replacing direct uses of the old bidirectional API in src/domain/text/last_word.rs and src/domain/text/convert.rs.
• Adjusted punctuation mappings so RU->EN and EN->RU behaviors are distinct (added complementary &↔? mapping) and added a unit test punctuation_only_respects_explicit_direction in src/tests/mapping_invariants_tests.rs.
• Documented required agent workflow checks (format, clippy, build, tests) in AGENTS.md and applied rustfmt-aligned formatting tweaks to a few modules.

Testing

• Ran cargo +nightly fmt --check which succeeded.
• Ran cargo +nightly clippy --all-targets --all-features -- -D warnings which failed due to numerous unresolved Windows API items (the windows crate / Win32 symbols are not available in the current CI/host environment), producing type and unresolved-symbol errors.
• Ran cargo +nightly build --features debug-tracing which failed for the same reason (many unresolved windows symbols and environment-specific items), so the project binary could not be built here.
• Ran cargo +nightly test --locked which also failed because the crate could not be compiled; the new unit test is present but could not be executed due to the build failures described above.

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Codex Task