Add base-platform-tools to workspace.yaml.example

[codeforester]

Context

docs/runtime-environment.md in the Base repository documents the optional base-platform-tools companion repository. When a sibling base-platform-tools checkout is detected, Base sets BASE_PLATFORM_TOOLS_HOME and BASE_PLATFORM_TOOLS_BIN_DIR and adds its bin/ directory to PATH after Base's own bin/. This is now part of the Base v1.5.0 runtime contract.

workspace.yaml.example currently lists base, base-demo, and base-bash-libs as the expected workspace shape. It omits base-platform-tools, so basectl workspace status --manifest workspace.yaml.example gives an incomplete picture of the reference workspace that Base intends users to have.

Scope

• Add a base-platform-tools entry to workspace.yaml.example with required: false (it is optional).
• Update the discovery_step in demo/demo.sh to note that base-platform-tools is an optional companion when it appears in workspace status output.
• No contract row needed unless a durable invariant is being added.

Validation

• basectl workspace status --manifest workspace.yaml.example lists base-platform-tools as an optional repo.
• The discovery_step in demo/demo.sh --non-interactive passes with the updated workspace example.