Do proper workspace scaffolding on pcb import

[akhilles]

Note

Medium Risk
Changes pcb import to create/git-init and template a full workspace when importing into a new directory, which affects filesystem writes and external git invocation and could impact import flows in CI/user environments.

Overview
pcb import now initializes a full workspace scaffold when the output directory is new (runs git init, writes pcb.toml, README.md, .gitignore, and installs the skill path) by reusing shared workspace initialization logic.

Workspace creation code is refactored to introduce init_workspace() for reuse, the workspace pcb.toml template now omits the repository field when empty, and E2E CI is updated to stop pre-creating a workspace before running import; release notes are added to CHANGELOG.md.

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