fix: preserve custom mise data for target tools

[brownie-ricon]

Finding fixed

PR #120 retains the caller's mise shims so the target project can discover its own tools, but then clears every MISE_* variable. A user-owned non-default MISE_DATA_DIR is required by those retained shims; clearing it makes them switch to the default data root, where they can fail offline or unexpectedly install a second copy.

This keeps MISE_DATA_DIR across the final child boundary while continuing to scrub Sessions project/task activation (MISE_CONFIG_ROOT, MISE_TASK_*, and the other inherited MISE_* values) plus usage_* and caller-CWD context. Bash and Elixir launch paths preserve the same rule.

Validation

• mise run test: 321/321 BATS tests pass
• mix test: 132 tests and 4 doctests pass
• readme build --check
• git diff --check

The updated real Pi-adapter engine test and Bash run-boundary test assert that the selected custom mise data root reaches the final child while task context remains absent.