Renderer: rebase relative links so source step docs work when viewed directly on GitHub

[pmalarme]

Problem

Step docs under .workshop/docs/steps/*.md render into the participant's root README.md, so their relative links and images are authored repo-root-relative (e.g. .workshop/docs/assets/01-*.png, .workshop/solutions/01-basic/). Those resolve correctly in the rendered README.

However, when a maintainer or participant opens a source step doc directly on GitHub (e.g. .workshop/docs/steps/01-basic.md), GitHub resolves relative links against that file's folder, so .workshop/solutions/01-basic/ becomes .workshop/docs/steps/.workshop/solutions/01-basic/ → 404. This predates the .workshop/ restructure (it was equally broken before at docs/steps/), but the restructure is a good moment to capture the fix.

This is the shared root cause behind a class of link bugs — e.g. stale docs/assets/... image links that only surfaced during the restructure review.

Current mitigation

Documented the root-relative convention in .github/instructions/workshop.instructions.md (source docs are authored for the rendered README; direct source viewing shows broken relative links by design). No behavior change.

Proposed enhancement (Option A)

1. Author source docs with source-relative links/images (../assets/..., ../../solutions/...) so they resolve when viewing the source doc directly on GitHub.
2. Add a rebasing pass in .workshop/scripts/render_readme.py that rewrites relative links/images from "relative to .workshop/docs/steps/" to "relative to repo root" when composing README.md. Must leave absolute URLs (https://...) and pure #anchor links untouched.
3. Add a lint rule in .workshop/scripts/lint_steps.py that rejects root-relative links in source docs (enforces the new convention), and ideally a link/image existence checker (which would also have caught the stale docs/assets/... links automatically).
4. Add renderer + lint tests covering: relative link rebasing, absolute-URL pass-through, anchor pass-through, and image paths.

Acceptance

• Source step docs render valid links both when viewed directly on GitHub and when inlined into the root README.md.
• Lint fails on root-relative links and on links/images that point at non-existent targets.
• python -m pytest .workshop/scripts/tests and python .workshop/scripts/lint_steps.py stay green.

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Migrated from pmalarme/foundry-workshop-template#69.