Document env vars for MCP dev mode #2112

Job	Run time
checks / readme-snippets	11s
checks / pre-commit	35s
checks / test (3.11, lowest-direct, ubuntu-latest)	53s
checks / test (3.10, locked, ubuntu-latest)	48s
checks / test (3.10, locked, windows-latest)	2m 25s
checks / test (3.12, lowest-direct, windows-latest)	3m 24s
checks / test (3.11, locked, windows-latest)	2m 40s
checks / test (3.13, lowest-direct, windows-latest)	2m 39s
checks / test (3.12, locked, windows-latest)	3m 28s
checks / test (3.10, lowest-direct, ubuntu-latest)	51s
checks / test (3.11, lowest-direct, windows-latest)	3m 4s
checks / test (3.12, locked, ubuntu-latest)	55s
checks / test (3.10, lowest-direct, windows-latest)	2m 48s
checks / test (3.11, locked, ubuntu-latest)	53s
checks / test (3.14, lowest-direct, windows-latest)	2m 37s
checks / test (3.14, locked, ubuntu-latest)	2m 29s
checks / test (3.12, lowest-direct, ubuntu-latest)	57s
checks / test (3.13, locked, ubuntu-latest)	47s
checks / test (3.14, locked, windows-latest)	3m 28s
checks / test (3.13, locked, windows-latest)	2m 26s
checks / test (3.13, lowest-direct, ubuntu-latest)	59s
checks / test (3.14, lowest-direct, ubuntu-latest)	1m 21s
all-green	3s
	40m 41s

Provide feedback