Document env vars for MCP dev mode #1992

Job	Run time
checks / readme-snippets	13s
checks / pre-commit	39s
checks / test (3.11, lowest-direct, ubuntu-latest)	57s
checks / test (3.10, lowest-direct, windows-latest)	3m 14s
checks / test (3.11, lowest-direct, windows-latest)	3m 9s
checks / test (3.11, locked, ubuntu-latest)	50s
checks / test (3.12, lowest-direct, windows-latest)	3m 41s
checks / test (3.10, locked, windows-latest)	2m 30s
checks / test (3.12, lowest-direct, ubuntu-latest)	59s
checks / test (3.13, lowest-direct, ubuntu-latest)	59s
checks / test (3.11, locked, windows-latest)	2m 35s
checks / test (3.12, locked, ubuntu-latest)	56s
checks / test (3.10, lowest-direct, ubuntu-latest)	56s
checks / test (3.14, lowest-direct, windows-latest)	2m 15s
checks / test (3.14, locked, windows-latest)	3m 29s
checks / test (3.12, locked, windows-latest)	3m 5s
checks / test (3.10, locked, ubuntu-latest)	12s
checks / test (3.13, lowest-direct, windows-latest)	2m 59s
checks / test (3.13, locked, ubuntu-latest)	45s
checks / test (3.14, lowest-direct, ubuntu-latest)	1m 26s
checks / test (3.13, locked, windows-latest)	2m 20s
checks / test (3.14, locked, ubuntu-latest)	2m 30s
all-green	5s
	40m 44s

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