feat: described slash picker for idle composer

[AetherAI3]

Closes #322.

Change

• The idle raw TTY composer shows a bounded slash picker from the command manifest. Rows include canonical names, argument hints, descriptions, and known static invocation support. Aliases collapse; hidden and managed-agent-scoped commands are omitted.
• Up/Down and Tab select. Enter inserts editable command text only; a later Enter submits. Escape restores the pre-picker draft and caret. Zero-match dismissal keeps literal slash text. Existing argument suffixes and caret positions survive acceptance.
• Bracketed paste, busy typeahead, PTY/setup/approval input, and line mode retain their input owners. Narrow and short terminals bound the panel, and resize redraws it.
• The coding slash dispatcher uses the same manifest scope decision for managed-agent handoffs. README, command help, and COMMANDS describe the picker.

Evidence

• npm run docs:check: passed (TypeScript build and six generated documentation outputs).
• Focused manifest, picker, composer, history-buffer, queue, shell, model, and slash fixtures: 103 passed, 0 failed, 4 platform skips.
• Raw TTY fixture proves selection is non-executing, second Enter executes, Escape returns keys to history recall, paste bypasses filtering, and editing makes zero model calls. Pure fixtures cover zero/one/many, aliases, scope, capability, suffix/caret, width/height, and resize.
• git diff --check: passed.
• PR Preserve exact shell review and queue authority on v4.21.0 main #309 integration check: src/commands/chat.ts, queue, and shell test edits auto-merge with its current draft head. docs/generated/commands.md is the sole conflict, and the same conflict exists between PR Preserve exact shell review and queue authority on v4.21.0 main #309 and pre-[feature] Commands: show a described slash picker while typing #322 main. This PR does not alter shell-review authority.
• Issue [feature] Console: search workspace prompt history with Ctrl+R #325's Ctrl+R search overlay is not in current main. This PR establishes picker precedence and tests the transition back to existing history recall; [feature] Console: search workspace prompt history with Ctrl+R #325 will need cross-overlay fixtures when its search UI lands.