docs: refactor docs structure and tighten assistant instruction policy

shrink root README into a landing page with a docs map and focused contributor guidance
add TV_POWER_RUNBOOK as the canonical TV power rollout and canary runbook
add CHANGELOG and move project history out of README-style docs
refactor src README into a developer-focused guide (architecture, runtime files, MQTT, debugging)
prune redundant older HDMI docs and keep a canonical HDMI_CEC_SETUP path
update copilot instructions to a high-signal policy format with strict anti-shadow-README design rules
align references across docs to current files, scripts, and TV power behavior

.env.template

@@ -41,6 +41,14 @@ CEC_TURN_OFF_DELAY=30        # Seconds to wait before turning off TV after last
 CEC_POWER_ON_WAIT=5          # Seconds to wait after power ON command (for TV to boot up)
 CEC_POWER_OFF_WAIT=5         # Seconds to wait after power OFF command (increased for slower TVs)
 
+# TV Power Intent (MQTT-based coordinated power control, Phase 1)
+# Controls how the display manager decides whether to use local CEC or server-issued intent.
+#   local  — ignore MQTT intents; all power decisions are local (safe default for rollout)
+#   hybrid — prefer MQTT intent when present and valid; fall back to local CEC if not
+#   mqtt   — MQTT intent is authoritative; local CEC only fires as last-resort guard
+# See README.md "TV Power Intent — Rollout Runbook" before changing from 'local'.
+POWER_CONTROL_MODE=local     # local | hybrid | mqtt
+
 # Optional: MQTT authentication (if your broker requires username/password)
 #MQTT_USERNAME=
 #MQTT_PASSWORD=