docs: add GitHub Copilot App harness and restructure harnesses

[GeekTrainer]

Summary

Adds a new GitHub Copilot App learning path (9 lessons) and restructures the existing harnesses so each opens with its own prerequisites lesson. The App path mirrors the Copilot CLI flow a learner already knows, then layers in the features unique to the app so beginners can follow a familiar arc.

Type of change

• Workshop content (lesson MDX, partials, images)
• Site shell (docs/ Astro + Starlight wrapper)
• Copilot configuration (.github/copilot-instructions.md, instructions, agents, skills)
• Repo housekeeping (CI, dependabot, README, license)
• Other:

Verification

• cd docs && rm -rf dist && npm run build succeeds (37 pages; page-count invariant holds at 36 routable mdx + 1 redirect)
• Lychee link check passes (0 errors)
• Partial guardrails pass (lint_partials.py, sync_partial_metadata.py --check)
• External GitHub URLs that I changed have been clicked manually (lychee runs offline)

Screenshots

The App lessons reference 11 captured screenshots under docs/src/content/docs/_images/app-*.png (template creation, session start, review panel, terminal, new-session-from-issue, Plan mode, change selection, enable Agent merge, and the Agent merge dropdown).

Notes for reviewers

What's new — the App harness (Ex 0-8): Prerequisites (install Node.js, create the repo from the template), install the app and start a session against the repo, add a star rating, guide Copilot with custom instructions from the seeded issue, build category/publisher filtering from the feature-request issue (running tests via the agent skill), manually test with the Playwright MCP server, merge with Agent Merge, plan with a shared Kanban canvas, and a closing review. The flow deliberately tracks the CLI path so the app-specific concepts (sessions-from-issues, Plan/Autopilot modes, Agent Merge, canvases) are the only new surface area.

Restructure: Every harness now opens with its own 0-prerequisites.mdx; the old shared prereqs.mdx becomes a full-HTML redirect (src/pages/shared/0-prereqs.astro) so the legacy URL still resolves and stays Pagefind-indexable. CLI and VS Code lessons were renumbered (git rename-tracked) to match. The Playwright MCP exercise is shared across paths, and the landing pages/sidebar were updated to surface each harness's core landing page first.

Surprising bits worth a look:

• The app _images/app-*.png set replaces an earlier orphaned set; 21 now-unused images were deleted in the same change.
• One unverified assumption: the seeded issue titles referenced in the App lessons (custom-instructions and filtering) were aligned to the backlog wording in Lesson 1. I could not confirm them against a freshly seeded template repo, since seeding happens via an external/org action rather than a workflow in the template. Worth a spot-check.
• The demo app the App lessons target lives on github-samples/tailspin-toys@geektrainer/reimagined-goggles (static Astro 7 + Drizzle/libSQL). This PR is content-only and adds no application code.