docs: initialize v0.8 to v1.0 migration guide skeleton

[tablackburn]

Phase 1 of #120. Initializes docs/migration-v0.8-to-v1.0.md so Phase 2+ breaking PRs have a defined home for migration entries and the v1.0.0 deliverable AI-assisted migration prompt has a stable, versioned location.

What's included

• docs/migration-v0.8-to-v1.0.md (new) — empty skeleton with:
  • Scope blurb (audience, non-scope link to CHANGELOG.md)
  • Quick Start placeholder ("no breaking changes have landed yet; this list will be populated by Phase 2 PRs")
  • AI-assisted migration prompt — referential style, designed for IDE/CLI agents (Claude Code, Copilot CLI/Chat, Cursor, Aider) that have file-system access. Tells the agent to read this guide + the user's build file and migrate it; flags uncertain spots with # MIGRATION-REVIEW: comments.
  • Migration entries section with one clearly-labeled illustrative example entry (fictional break) demonstrating the loose format. Wrapped in HTML comments instructing the first real Phase 2 PR to delete it.
  • Adding an entry spec for PR contributors — loose format conventions modeled on psake/psake docs/migration-v4-to-v5.md: heading + prose + Before/After code blocks, with the rigid 5-field template downgraded to suggested subsections.
  • Related links to [Tracking] PowerShellBuild v1.0.0 roadmap #120 and the sibling-repo convention.
• CHANGELOG.md — one-line entry under ## Unreleased / ### Added.

Org convention alignment

Path diverges from #120's original spec:

• [Tracking] PowerShellBuild v1.0.0 roadmap #120's original: docs/migration/v0.8-to-v1.0.md (nested in a migration/ subdirectory)
• This PR: docs/migration-v0.8-to-v1.0.md (flat)

Reason: psake/psake already ships a sibling-pattern migration guide at docs/migration-v4-to-v5.md. Matching the org convention reduces cross-repo friction. #120's body has been updated to reference the new path.

Scope

Docs-only — no code, dependency, or workflow changes. The illustrative entry is fictional and gets deleted by the first real Phase 2 PR.

Not in this PR

• instructions/git-workflow.instructions.md — left alone (upstream AIM template). The "Adding an entry" spec for PR authors lives inside the migration file itself.
• Real migration entries — added by Phase 2 PRs as breaks land.

Test plan

• CI passes (existing required checks)
• Visual review on GitHub: prompt block renders as a code block, entry headings anchor cleanly
• Confirm flat path matches the psake/psake convention

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