docs(plans): record the gacha wish command plan

plans/261001-1052-gacha-wish-command/plan.md

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+# /gacha wish command
+
+Status: done
+
+## Outcome
+
+`/gacha <option[*rarity],...>` picks one option and replies with a Genshin
+Impact style wish animation (meteor coloured by rarity, white flash, reveal
+with stars), rendered as a silent MP4 by the existing wheelofnames service.
+
+## Decisions (user-approved 2026-10-01)
+
+- Renderer lives in `tiennm99/wheelofnames` as `POST /api/gacha`; no new repo
+  or deployment.
+- Rarity is user-tagged: `Pizza*5, Pho*4, Rice` (untagged = 3★).
+- Single pull only.
+- Output MP4 (H.264, no audio) sent via `sendAnimation`.
+
+## Design
+
+- Odds follow Genshin base tier rates, normalised over tiers present:
+  5★ 0.6%, 4★ 5.1%, 3★ 94.3%; uniform within a tier. All-untagged input is
+  therefore a plain uniform pick.
+- All visuals are procedural (CSS/gradients); no HoYoverse assets.
+- Bot derives the gacha endpoint from `WHEELOFNAMES_API_URL` (sibling path
+  `gacha` next to `gif`), so no new env var. Same bearer token.
+- Bot is the source of truth for winner and rarity; renderer only draws.
+- Fallback without renderer or on failure: text reply `★★★★★ Pizza`.
+
+## Phases
+
+1. Renderer: schema, `GachaComposition`, MP4 render, route, tests, docs.
+2. Bot: parser, weighted pick, API client, command, tests, README/docs.
+3. Verify: renderer stills via Docker, lint/typecheck/test, go test/vet/lint.
+
+## Acceptance
+
+- Each rarity renders with the correct meteor/glow colour and star count.
+- `/gacha` with no options shows usage; untagged options behave like /random.
+- Renderer failure falls back to text; thread IDs forwarded.