Add feature-notebook workflow: per-feature design specs + test results feeding PRs

[andrewjong]

Motivation

Coding-agent sessions accumulate valuable context — the problem framing, design decisions, and validation evidence — that evaporates when the conversation ends. This PR adds a feature notebook workflow: every feature an agent implements gets a numbered entry under notebook/ at the repo root that records the design spec before coding and the test results after, and that content is what populates the feature's PR description.

What changed

New skill: .agents/skills/use-feature-notebook/

• SKILL.md — the workflow:
  1. Before coding: create notebook/NNN-feature-slug/design_spec.md — problem context from the session, proposed implementation, and a lettered test plan (a), (b), …
  2. While implementing: keep the spec's status labels current — an overall status in the header plus a DESIGN/TODO / WIP / DONE label on each implementation subsection
  3. During validation: store raw artifacts under results/<letter>-<section-slug>/, letters matching the spec's test-plan sections
  4. After validation: write results/results_summary.md — self-contained, with quantitative tables and qualitative figures embedded directly, plus per-section verdicts
  5. On opening the PR: populate the PR body from the spec (motivation, what changed) and the results summary (validation tables, figures as attachments)
• assets/design_spec_template.md and assets/results_summary_template.md — fill-in templates

AGENTS.md

• New use-feature-notebook row in the skills registry
• Agent Workflow Example now starts with the notebook entry and ends with the results summary → PR step
• New "Feature Notebook (notebook/)" section with the directory layout and the local-only rule

Docs site

• New page docs/development/intermediate/feature_notebook.md under Development → Intermediate Tutorials → Contributing (wired into mkdocs.yml nav and listed in the Development index)

.gitignore

• /notebook/ — the notebook is local-only on each developer's machine; its content leaves the machine only via PR descriptions

Version

• .env VERSION 0.19.0-alpha.9 → 0.19.0-alpha.10, CHANGELOG entry under [Unreleased] (docs/tooling-only change; bump clears the version gate)

Validation

• git check-ignore -v notebook/001-test → matched by the new /notebook/ rule
• Docs-only change: no Docker image content, code, or test behavior affected

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