[codex] Add slash command registry foundation

[roackb2]

Summary

This adds the M2 slash-command foundation without wiring it into the existing TUI command path yet.

Changes:

• Add core slash command types for parsed input, commands, modules, hints, and matches.
• Add a parser that rejects non-command input and absolute Unix paths.
• Add a registry that composes command modules, supports command-owned match predicates, and rejects duplicate module ids, command ids, syntaxes, and aliases.
• Add autocomplete helpers for shared-prefix completion and placeholder stripping.
• Add unit coverage for exact, alias, prefix-style matching, duplicate detection, parsing, and autocomplete behavior.

Impact

This prepares M3+ to migrate command behavior out of src/cli/chat/state/local-commands.ts incrementally, while preserving the existing TUI command behavior for now.

Validation

• yarn test:unit src/__tests__/unit/core/slash-commands.test.ts
• yarn test:unit src/__tests__/unit/tui/local-commands.test.ts
• yarn eslint
• yarn typecheck