feat(keys): hint mode and plugin keyboard shortcuts

[Rfluid]

Summary

Keyboard control for plugin panels, in two layers:

• Hint mode (f, action hint_mode). Every button in a plugin's controls section gets a label (a, s, d… home row first; fixed length so none is a prefix of another). Typing a label presses the button. Confirm buttons arm on the first press and fire on the second. Esc, a click, or any tab/plugin/mode switch leaves hint mode. Works with every existing plugin; no protocol change.
• Plugin keys. Sections can declare keys: [{ keys, action, label, confirm? }]. Each key sits behind a leader ([keybindings] plugin_leader, default space), so a plugin can never shadow an Aura shortcut. Pressing the leader opens a floating panel at the bottom right listing the section's keys; it narrows as strokes are typed. It waits until the shortcut completes or Esc, or for [keybindings] leader_timeout_ms when set (unset by default).

Design and decisions: docs/plans/plugin-keymaps.md.

How it works

• Hint mode and leader mode drop the root's keymap contexts, so no binding matches and the root's key_down listener reads the keys. Only the leader itself is a GPUI binding (new plugin context between global and overlay). Aura matches the rest with GPUI's KeyBinding::match_keystrokes instead of binding whole sequences, because GPUI forgets a pending sequence after a fixed second, and Esc during one replayed into dismiss.
• Clicks, hint labels and plugin keys share one press path (press_plugin_action), so confirm behaves the same everywhere. Esc (and q) disarm an armed button before closing anything.
• Key resolution lives in aura-core (plugin/keys.rs): the leader goes in front, spellings are canonicalized, bad entries are skipped with warnings, duplicates keep the later entry, and a key that starts another is flagged (the shorter one runs immediately).
• Remapping plugin keys is left to plugins; Aura installs what the panel declares.

Also in this PR

• fix(plugin): aura plugin add --link now links to the absolute source path. A relative source (as install.sh --link passes) left a dangling symlink inside the plugins dir, and discovery silently skipped the plugin.
• aura keys validate now prints its warnings when keybindings.toml doesn't exist (it used to exit 1 with no explanation).
• CLI: aura plugin run prints each section's resolved keys and warnings on stderr; aura keys validate and aura doctor warn when the leader collides with a [global] binding.
• Docs: keybindings.md, plugin-authoring.md, configuration.md, .design/components.md.

Testing

• ./scripts/pre-pr.sh: all checks pass (rustfmt, clippy, test, selectable_text, gitleaks, cargo_audit, build_release).
• New unit tests: hint labels/matching, key resolution and leader warnings, PluginSection.keys round-trip, absolute symlink target.
• Manually tested in the modal against the Audio Hooks plugin (companion PR in Rfluid/aura-audio-hooks): hint mode, space panel, confirm keys.