Fix Base self-demo to use the owning runtime instead of PATH basectl

[codeforester]

Goal

Make the documented Base self-demo invoke the matching Base runtime and dependency set, even when an older globally installed basectl appears earlier on PATH.

Background

At review commit 7ac42b2, the documented command ./bin/basectl demo base -- --non-interactive failed in a normal mixed-install environment with:

Base requires base-bash-libs 1.3.0 or a compatible later 1.x release; loaded version is "2.0.0".

The failure is caused by demo/demo.sh, whose #!/usr/bin/env basectl shebang resolves a globally installed executable before the repository-owned runtime can take control. The self-demo then runs commands through bare basectl and base-wrapper names as well. The full unit and BATS suites pass, so the current tests do not cover this mixed-install integration boundary.

Scope

• Remove or replace the Base self-demo interpreter path that can resolve an unrelated global basectl.
• Ensure nested basectl and base-wrapper calls use the owning Base checkout or installed Base home consistently.
• Preserve the generic basectl demo <project> contract for external project-owned demos.
• Add a regression fixture or smoke test with an older fake/global basectl earlier on PATH.
• Update the self-demo documentation and captured proof if the invocation or output changes.

Acceptance Criteria

• The documented self-demo command succeeds from a clean source checkout when an incompatible basectl is earlier on PATH.
• Every command executed by the Base self-demo uses the same Base home, Bash library version, and Python CLI source expected by the outer invocation.
• The failure cannot silently fall back to a globally installed Base version.
• A regression test proves the mixed-install case and passes in the supported local gate.
• The self-demo remains non-interactive and does not require the external base-demo repository.

Validation

• Run the focused self-demo BATS tests with the fake incompatible executable on PATH.
• Run ./bin/basectl demo base -- --non-interactive from the source checkout.
• Run the Base demo E2E or equivalent packaged/source integration path.
• Run env -u BASE_HOME ./bin/base-test.
• Run git diff --check.

Non-Goals

• Do not change the supported base-bash-libs compatibility policy.
• Do not redesign the generic project demo feature.
• Do not make the self-demo depend on GitHub, network access, or the external base-demo repository.

Project Fields

• Status: Backlog
• Priority: P0
• Size: S
• Area: Runtime
• Initiative: Adoption Polish

Agent Assignment

Human first. This is a release-facing runtime boundary and should be reproduced and reviewed before implementation.