feat(shortcuts): default to hold Right Alt (push-to-talk) for new installs (#648)

* feat(shortcuts): default to hold Right Alt push-to-talk on first install

Double-tap Ctrl conflicts with many desktop and app shortcuts, so new
installs now use hold Right Alt (Option) in push-to-talk mode. Existing
config.json values are preserved unchanged.

Updates README, user guide, AGENTS.md, first-run dialog, website copy,
and tests to match the new defaults.

* refactor(shortcuts): trim redundant default-change test and comments

Drop the fresh-install assertion that duplicated DEFAULT_CONFIG coverage,
remove the duplicated Right-Alt rationale from config_manager, and drop a
vacuous source-string check from the settings shortcuts test.

* fix(shortcuts): align install.sh seeds and preserve legacy toggle mode

install.sh still wrote ctrl+ctrl without mode, so merges with the new
push_to_talk default became hold-Ctrl. Seed right_alt + push_to_talk for
new installs, and migrate existing shortcut-only configs to mode=toggle
so prior behavior is unchanged.

* style(config): fix black formatting after shortcuts migration

* fix(shortcuts): preserve legacy defaults when shortcuts section is missing

Existing config files without a shortcuts section previously inherited
ctrl+ctrl toggle from DEFAULT_CONFIG. Pin those historical defaults on
upgrade so the new first-install Right Alt push-to-talk defaults do not
silently change behavior.

* docs(agents): unwrap hard-broken dictation control paragraph

Keep the AGENTS.md paragraph as a single line like the surrounding prose.

tests/test_evdev_backend_ext.py

@@ -212,8 +212,8 @@ class TestEvdevKeyboardBackendInit:
     def test_init_default_values(self):
         """Test initialization with default values."""
         backend = EvdevKeyboardBackend()
-        assert backend.shortcut == "ctrl+ctrl"
-        assert backend.mode == "toggle"
+        assert backend.shortcut == "right_alt+right_alt"
+        assert backend.mode == "push_to_talk"
         assert backend.active is False
         assert backend.devices == []
         assert backend.device_fds == []