Make Command Help data-driven from plugin commands and remove dashboard tip

[multiplex55]

Motivation

• Replace the hard-coded help example table with data-driven content so new plugin commands automatically appear in help.
• Surface full plugin information in the Command Help dialog including description, capabilities, prefixes, usage and examples derived from plugin metadata.
• Remove the unrelated Dashboard tip from the Command Help dialog so the window opens directly into command content.
• Improve filtering so it matches plugin name, description, capabilities and command labels/actions.

Description

• Stop using the static example_queries() table and instead derive help from each plugin's commands() returned Actions by collecting app.plugins.plugin_infos() and plugin.commands().
• Render capabilities, computed prefixes (from command labels), usage (command labels), and examples (from Action.action prefixed with query: or fallback to Action.label) in the help UI.
• Add helper functions matches_filter, command_prefixes, and example_line and import Action and HashSet in src/help_window.rs to implement filtering and presentation logic.
• Remove the dashboard tip label from the Command Help window and adjust layout to show per-plugin sections sorted by name.

Testing

• No automated tests were run for this change.
• The change was compiled and committed as part of the rollout (no test suite executed).

---

Codex Task