docs: add /memory shortcut to README

[owainlewis]

Summary

• add the /memory <text> slash command to the README TUI shortcuts table
• add a short shortcut example showing that /memory appends a project memory entry

Verification

• git diff --check
• rg -n "(/memory |append a project memory entry|resume a saved session|select permission mode|list available tools)" README.md internal/tui/info.go docs/developer/guides/memory.md
• subagent review against issue Document /memory in the README TUI shortcuts #126 acceptance criteria (no findings)