docs: 'how to use' explainer GIF

[dvd90]

Adds docs/usage.gif — a short terminal-styled explainer that walks through using gitgotchi, embedded near the top of the README so the usage reads at a glance.

The GIF cycles through:

1. what Byte is (ambient, not a linter)
2. gitgotchi — one check-in
3. gitgotchi watch — live view (with an idle blink)
4. how the repo drives the mood (commit → fed, TODOs → grubby, CI red → sick)
5. gitgotchi card — share it

Rendered from the real sprite art with a monospace font (no external recorder available), 60 kB, loops.

🤖 Generated with Claude Code

[claude]

Claude finished @dvd90's task in 1m 14s —— View job

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Review of PR #7 — docs: 'how to use' explainer GIF

• Gather context (diff, CLAUDE.md conventions)
• Review against CLAUDE.md conventions (TDD, engine purity, collectors, product principles, milestone scope)
• Post findings

Summary: This is a docs-only change — README.md (+6/-0) and a new binary docs/usage.gif (60,685 bytes). No src/** files are touched, so the TDD gate, engine purity (src/engine/**), and "collectors never throw" (src/collectors/**) rules don't apply here — there's no new behavior to cover with tests. Scope is tight and matches "one milestone per PR" in spirit (this isn't tied to a milestone at all, which is appropriate for a docs asset).

Checks performed:

• ✅ The GIF's claimed walkthrough (gitgotchi, gitgotchi watch, gitgotchi card) matches real, implemented CLI commands (src/cli.ts:56-91) — not documenting vaporware.
• ✅ Alt text (README.md:22) is descriptive and accessible, and matches the product principle of dry/factual copy — no guilt-tripping language.
• ✅ Placement is sensible: right after the quick-start snippet, before the existing "Contents" TOC.
• ✅ Size (~59 KB) is reasonable for a looping GIF; no LFS config exists in the repo but this is in line with the pre-existing docs/demo.gif (32 KB), which is also committed directly as a binary.

Minor, non-blocking nit:

• The new ## How to use — a 30-second tour section isn't added to the ## Contents list right below it (README.md:25-33), which links every other top-level section. Consider adding an anchor entry for consistency, though it's cosmetic.

No issues with product principles, architecture rules, or milestone scope. Looks good to merge as-is.