Fix Base self-demo to use the owning runtime

[codeforester]

Summary

• run the Base self-demo through the checked-out runtime that owns the script
• keep explicit test overrides available for hermetic fixtures
• add regression coverage for PATH shadowing by an incompatible basectl

Issue

Fixes #2081

Validation

• BASE_BASH_LIBS_DIR=/Users/rameshhp/work/base-bash-libs/lib/bash bats cli/bash/commands/basectl/tests/demo.bats (12 passed)
• shellcheck demo/demo.sh
• env -u BASE_HOME BASE_BASH_LIBS_DIR=/Users/rameshhp/work/base-bash-libs/lib/bash BASE_CLI_SOURCE_DIR=/Users/rameshhp/work/base-cli/lib/python BASE_CACHE_DIR=/private/tmp/base-pr-train-2081 ./bin/base-test (1,115 pytest tests and 900 BATS tests passed)
• git diff --check
• basectl demo base --dry-run -- --non-interactive

Demo Impact

The documented self-demo now resolves nested basectl and base-wrapper calls from the same Base checkout as demo/demo.sh, so an incompatible global basectl cannot intercept the run. Explicit BASE_DEMO_BASECTL and BASE_DEMO_BASE_WRAPPER overrides remain available to tests and integrators.

Notes

The real demo remains trust-gated by the existing manifest approval boundary; validation did not persist a user-owned trust grant. .ai-context/ is unchanged because the public command shape and product architecture are unchanged.