docs: showcase packages with screenshots and demo media

[NachoVazquez]

TL;DR

Every package README was text-only — six solid products with zero visual proof. This PR gives each README a hero image or demo GIF, real screenshots captured from live Pi/OpenCode/CLI sessions, and a landing-page structure (hero → install → what you get → details), with all factual content preserved.

Files to review (15, +330 / −368):

File	Why
docs/media/** (new media, start here)	8 assets: Pi card/footer/waveform GIF, OpenCode sidebar/compact, vim -- INSERT --, apnea CLI GIF.
packages/pi-music-dock/README.md	Biggest rewrite: hero, chip feature documented for the first time, commands/behavior kept.
packages/music-core/README.md	Adds mermaid architecture diagram and a verified stepEngine/displayLevel example.
packages/apnea/README.md	CLI GIF hero + "why panes instead of subagents" section; reference tables untouched.
README.md	Root package table now carries thumbnails.

How

• Media lives in docs/media/<package>/, outside every npm tarball. All six verify-pack exact-match gates still pass. READMEs reference assets via raw.githubusercontent.com URLs so npmjs.com renders them; they resolve once this merges to main (they 404 on this PR until then).
• Screenshots are real. Pi card captured from an isolated PI_CODING_AGENT_DIR running local 0.1.2 with tokyo-night-moon; OpenCode sidebar/compact from a live session; GIFs generated with VHS (apnea help/status, and the waveform engine's playing state).
• No facts invented. Every command, requirement, and option table is carried over from the previous READMEs; new claims (chip behavior, seek click regions, tool names) were checked against source.

Reviewer notes

• Private data is excluded. The herdr workspace screenshot was rejected — it showed session spend and ticket links. Every shipped crop was checked for paths, spend, and identifiers.
• displayLevel(level, index, playing) in the new music-core example matches the real signature, not the engine-taking guess.
• Root README thumbnails use HTML <img>/<a>, not markdown, because GitHub does not parse markdown inside HTML blocks.

Tests

bun run format:check and bun test scripts pass; all six pack:check gates verified. No product code changed.