Retire legacy Dev Mode orchestration

[fredporter]

Outcome

• removes the duplicate dev-mode-executor orchestration skill
• removes the destructive dev-destroy-rebuild skill
• removes the redundant autostart skill and MCP tool
• changes startup/watchdog integration to the canonical health endpoint
• removes the archived monolithic MCP handler and stale feature/status specs
• preserves the actively used Developer display-mode toggle
• aligns fallback capability samples with surviving IDs

Verification

• backend: 502 passed, 6 existing aiohttp warnings
• focused MCP/registry: 35 passed
• Ruff: passed
• docs non-regression: passed