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

TV_POWER_HANDOFF_CLIENT.md

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+# Client Handoff: TV Power Coordination
+
+## Purpose
+Implement robust client-side TV power control that applies server MQTT intents when valid and falls back to local event timing when server/broker data is missing or stale.
+
+## Source of Truth
+- Shared full plan: TV_POWER_COORDINATION_TASKLIST.md
+
+## Scope (Client Team)
+- Intent subscription/validation
+- CEC state transitions and timer cancellation safety
+- Hybrid fallback using local event windows
+- Power state acknowledgment publishing
+
+## MQTT Contract (Client Responsibilities)
+
+### Subscribe
+- infoscreen/{client_id}/power/intent
+- Optional: infoscreen/groups/{group_id}/power/intent
+
+### Publish state
+- infoscreen/{client_id}/power/state
+
+### State Payload (v1)
+```json
+{
+  "schema_version": "1.0",
+  "intent_id": "last-applied-intent-id",
+  "client_id": "...",
+  "reported_at": "2026-03-31T12:00:01Z",
+  "power": {
+    "applied_state": "on",
+    "source": "mqtt_intent|local_fallback",
+    "result": "ok|skipped|error",
+    "detail": "free text"
+  }
+}
+```
+
+## Required Runtime Rules
+
+### Intent Validation and Ordering
+- Accept only schema_version=1.0 (or explicitly version-gated supported set).
+- Ignore malformed payloads.
+- Ignore expired intents (expires_at in past).
+- Apply only newest valid intent by issued_at, intent_id tie-break.
+- Deduplicate already-applied intent_id.
+
+### Power Action Safety
+- desired_state=on:
+  - cancel pending delayed-off timer immediately
+  - turn on via CEC only if needed
+- desired_state=off:
+  - schedule delayed off (grace_seconds or local default)
+  - re-check active event before executing actual off
+
+### Fallback (Critical)
+- If MQTT unavailable, intent missing, invalid, or stale:
+  - use existing local event start/end logic
+  - use event end as off trigger plus delayed-off safety
+- Any active event must cancel pending off timers.
+
+## Configuration
+- Add POWER_CONTROL_MODE with values:
+  - local
+  - mqtt
+  - hybrid (recommended default)
+
+### Hybrid Mode
+- Prefer valid MQTT intent.
+- Automatically fall back to local schedule logic when intent channel is not trustworthy.
+
+## Implementation Tasks
+1. Add intent topic handlers and schema validation.
+2. Integrate intent application into display power control path.
+3. Add timer race hardening for adjacent event transitions.
+4. Add fallback decision branch for stale/missing intents.
+5. Add power state publisher with intent_id/source/result.
+6. Add logs for timer arm/cancel/fire with reason and event_id.
+7. Add tests for adjacent events, broker outage, reconnect, duplicate intent.
+
+## Acceptance Criteria
+1. No unintended TV off between adjacent events.
+2. Broker outage during active event does not power off TV prematurely.
+3. Reconnect with retained intent reconciles state without oscillation.
+4. Duplicate/old intents do not trigger repeated CEC toggles.
+5. State messages clearly show mqtt_intent vs local_fallback source.
+
+## Target Integration Points
+- Main runtime orchestration: src/display_manager.py
+- MQTT plumbing and topic handlers: src/simclient.py
+
+## Operational Notes
+- Keep fallback logic enabled even after MQTT rollout.
+- Ensure all new timestamps are UTC ISO format.