[codex] Refactor session slash command module

[roackb2]

Summary

Refactors the M4 session slash-command slice so /continue, /clear, and the /session ... command family run through the core slash command registry instead of the TUI local command switchboard.

Changes include:

• Added src/core/commands/slash/modules/session/session-commands.ts for session command behavior and module-owned static hints.
• Added session ports to SlashCommandExecutionContext and wired them from the TUI slash command adapter.
• Removed session command handlers and static session hints from local-commands.ts; dynamic /session switch <id> hints remain in the TUI wrapper because that API receives live session state.
• Added direct core module tests for session host-port routing, required-argument matching, and pure session reference resolution.

Impact

This keeps existing TUI behavior stable while moving session command policy into a reusable core module. Session reference resolution now has a dedicated pure helper and preserves the existing exact-id-before-index behavior.

Validation

• yarn test:unit src/__tests__/unit/core/slash-command-modules.test.ts
• yarn test:unit src/__tests__/unit/tui/local-commands.test.ts
• yarn vitest run src/__tests__/integration/tui/session-cli.test.ts
• yarn eslint
• yarn typecheck
• yarn test outside the sandbox, because the full integration suite needs localhost binding and normal git identity for temp repositories