Document env vars for MCP dev mode #1994

Job	Run time
checks / pre-commit	35s
checks / readme-snippets	11s
checks / test (3.10, lowest-direct, windows-latest)	2m 59s
checks / test (3.11, lowest-direct, ubuntu-latest)	1m 0s
checks / test (3.10, lowest-direct, ubuntu-latest)	59s
checks / test (3.10, locked, windows-latest)	3m 47s
checks / test (3.10, locked, ubuntu-latest)	56s
checks / test (3.11, locked, ubuntu-latest)	52s
checks / test (3.11, lowest-direct, windows-latest)	2m 57s
checks / test (3.12, locked, ubuntu-latest)	1m 1s
checks / test (3.12, lowest-direct, windows-latest)	3m 27s
checks / test (3.12, lowest-direct, ubuntu-latest)	1m 2s
checks / test (3.12, locked, windows-latest)	3m 29s
checks / test (3.14, locked, ubuntu-latest)	2m 23s
checks / test (3.13, lowest-direct, windows-latest)	2m 42s
checks / test (3.11, locked, windows-latest)	2m 29s
checks / test (3.13, lowest-direct, ubuntu-latest)	56s
checks / test (3.13, locked, ubuntu-latest)	50s
checks / test (3.13, locked, windows-latest)	2m 29s
checks / test (3.14, locked, windows-latest)	3m 58s
checks / test (3.14, lowest-direct, ubuntu-latest)	1m 21s
checks / test (3.14, lowest-direct, windows-latest)	2m 30s
all-green	4s
	42m 57s

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