release: cut v0.5.0 with README refresh, CI doc validation, and changelog finalization

[ofriw]

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Release: v0.5.0

This branch cuts v0.5.0. The actual feature work (frontmatter model selection, transactional discardPages, Model Groups/spawn improvements) landed in earlier merges; this PR finalizes the release — bumps the version, makes CI validate documentation, completes the changelog, and rewrites the README to lead with the workflow story instead of a primitive list.

What v0.5.0 delivers (the value)

• Frontmatter-driven model selection — skills/prompts declare model, model-group, and thinking in frontmatter so the procedure owns its model policy, not runtime steering.
• Handoff discardPages — prune stale notebook pages transactionally (epoch-stamped, committed only after compaction succeeds); interrupted discards keep pages.
• Model Groups editing & spawn routing — searchable pickers, inline group-name editor, #group autocomplete with model/thinking details, max thinking, single disposal across all lifecycle races.

Changes on this branch

Added

• 0.5.0 changelog section with two new Added entries (frontmatter model selection, handoff discardPages).
• CI badge + standard npm badge in the README header.

Changed

• Version 0.4.0 → 0.5.0 (package.json + package-lock.json); dev dep @earendil-works/pi-ai pinned to 0.84.1.
• CI no longer path-ignores root *.md — README/CHANGELOG edits now run the test workflow (**/docs/** stays ignored).
• README restructured from a feature/primitive list into a workflow-example narrative, with a concrete repeatable-review example and an expanded compatibility/limits table.
• Documented minimums corrected to Pi 0.84.1 and Node 22.19.0.

Removed

• README "Why / Features / Comparison / Primitives" sections (folded into the new narrative); the static status: active badge.

Breaking / compatibility

• Requires Pi 0.84.1+ and Node 22.19.0+. Spawn depends on Pi's public selected-model and child-owned runtime APIs — older engines will not satisfy the contract. This is the only hard floor; behavior is otherwise backward compatible.

Why it matters

The package's pitch shifts from "context tools" to a composable workflow layer: encode the procedure once (model role, memory, permissions, boundaries) and repeat it reliably across changesets, with models swapped via Model Groups without touching the workflow. This release documents and ships that story end-to-end.

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Attached is an agent optimized description of the changes in this PR - AGENT_REVIEW.md