Add text_command + refresh live-text keys

Introduce a per-key `text_command` + `refresh` feature: the service
periodically runs a shell command and renders its stdout as the key's
text overlay, compositing over the base icon (or a black background for
text-only keys) each tick. `text` acts as the static fallback shown
before the first run or when the command errors/outputs nothing. A
refresh goroutine owns the key's image and also handles an optional poll
block (choosing icon_true/icon_false per tick). Adds `$(...)` sugar in
`text` as shorthand for `text_command`.

- internal/config: add TextCommand/Refresh fields to KeyConfig
- cmd/streamdeck: refreshTextKey goroutine, runTextCommand, unwrapCmdSubst
- config.example.yaml: document the feature with a `flow` pomodoro
  live-countdown example (status --short, toggle/skip/stop/gui)
- modules.example.yaml: example config updates

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01TEoLyNbtbjbDq7aTWAymUG

internal/config/config.go

@@ -27,6 +27,12 @@ type KeyConfig struct {
 	TextColor string `yaml:"text_color"` // text color: "white" (default), "black", "red", "blue", or hex "#RRGGBB"
 	Command   string `yaml:"command"`    // shell command to run on press
 
+	// Dynamic text: periodically run TextCommand and render its stdout as the
+	// key's text overlay. Refresh is the interval (default 1s, floor 250ms).
+	// Text (above) is the static fallback / initial value.
+	TextCommand string `yaml:"text_command"` // shell command whose stdout becomes the overlay text
+	Refresh     string `yaml:"refresh"`      // how often to re-run text_command, e.g. "1s" (default: "1s")
+
 	// Toggle/status keys: show different icons based on polled state.
 	IconTrue  string      `yaml:"icon_true"`  // icon when poll match is true
 	IconFalse string      `yaml:"icon_false"` // icon when poll match is false