Context key and `when` practice

Roadmap item 1.2.2 introduces shared context-key contracts and deterministic when expression evaluation. Follow these rules when changing context-aware command or keybinding behaviour:

• Define context-key and when AST contracts in shared/src/types/context-keys.ts and import those contracts through @shared/types/context-keys.
• Keep parser grammar constrained to the documented operators in docs/velocetty-design.md (&&, ||, !, ==, !=, <, <=, >, >=, and parentheses).
• Use lib/context-key-service.ts for runtime context-key management and expression evaluation, and reuse compiled expressions (compile(...)) when evaluating the same expression repeatedly.
• Treat context values as explicit primitives (boolean, string, number, or null) and avoid ad-hoc component booleans outside the context-key service.
• Add or update unit coverage in test/unit/context-key-service.test.ts for all operators, precedence/grouping, parse failure indices, deterministic repeated evaluation, and parser edge cases such as empty expressions, stray operators, identifier variants, and string-escape handling.