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

CHANGELOG.md

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+# Changelog
+
+## April 2026
+
+- Added Phase 1 TV power coordination on `infoscreen/groups/{group_id}/power/intent`.
+- Added `POWER_CONTROL_MODE` with `local`, `hybrid`, and `mqtt` behavior.
+- Added `src/power_intent_state.json` and `src/power_state.json` for power IPC and telemetry.
+- Added `infoscreen/{client_id}/power/state` publishing from `simclient.py`.
+- Added turn-off guard logic to avoid unintended TV-off races at event boundaries.
+- Added [TV_POWER_RUNBOOK.md](TV_POWER_RUNBOOK.md) and test tooling in `scripts/test-power-intent.sh`.
+
+## March 2026
+
+- Hardened event-trigger screenshots (`event_start`, `event_stop`) against periodic overwrite races.
+- Improved `latest.jpg` and `meta.json` synchronization for reliable dashboard updates.
+- Added self-healing for stale or invalid pending screenshot trigger metadata.
+- Improved display environment fallbacks (`DISPLAY=:0`, `XAUTHORITY`) for non-interactive starts.
+- Allowed periodic idle captures in development mode so dashboard previews stay fresh without active events.
+- Added content-type-aware trigger delays for event screenshots.
+- Changed screenshot transmission to a 1-second polling tick so triggered sends fire within <=1s.
+- Migrated dashboard payload to grouped schema v2 (`message`, `content`, `runtime`, `metadata`).
+
+## November 2025
+
+- Implemented the two-process screenshot pipeline (`display_manager.py` capture, `simclient.py` transmission).
+- Added Wayland/X11 screenshot tool fallback chains.
+- Extended dashboard payloads with screenshot and system metadata.
+- Extended scheduler event type support for `presentation`, `webuntis`, `webpage`, and `website`.
+- Added website autoscroll support via CDP injection and extension fallback.