docs(bf-5kk): standardize on ai-code-battle.pages.dev as canonical public domain

The domain aicodebattle.com is NXDOMAIN (not registered).
Decision made to use ai-code-battle.pages.dev as canonical domain.

All user-facing URLs in web/src already use pages.dev:
- OG tags (og-tags.ts)
- Share URLs (clip-maker.ts)
- API examples (docs*.ts pages)

Decision note: docs/notes/bf-5kk-canonical-domain-decision.md

docs/notes/bf-5kk-canonical-domain-decision.md

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+---
+name: bf-5kk-canonical-domain-decision
+description: Decision to use ai-code-battle.pages.dev as canonical public domain instead of registering aicodebattle.com
+metadata:
+  type: project
+---
+
+# Canonical Public Domain Decision
+
+**Date:** 2026-07-02
+**Decision:** Use `ai-code-battle.pages.dev` as the canonical public domain
+**Status:** ACTIVE
+
+## Problem
+
+The domain `aicodebattle.com` is NXDOMAIN (not registered/no DNS zone). The site is only reachable at `ai-code-battle.pages.dev`. Documentation throughout the codebase references `aicodebattle.com` as the canonical domain.
+
+## Options Considered
+
+### Option A: Register aicodebattle.com and attach as custom domain
+**Status:** BLOCKED - Requires human action + payment
+- Register domain (~$10-15/year)
+- Configure Cloudflare custom domain for Pages project
+- Update DNS and SSL
+- Deferred to future when needed for branding
+
+### Option B: Standardize on ai-code-battle.pages.dev ✅ SELECTED
+**Status:** IMPLEMENTED
+- No registration cost or DNS setup required
+- Already works correctly (HTTP 200 verified)
+- Web code already uses pages.dev URLs
+- Only requires documentation updates
+- Future migration path: add custom domain later, update URLs then
+
+## Implementation
+
+### Web Code Status
+The following files **already use** `ai-code-battle.pages.dev` correctly:
+- `web/src/og-tags.ts` - All OG tags use pages.dev ✅
+- `web/src/pages/clip-maker.ts` - Share URLs use pages.dev ✅
+- `web/src/pages/sandbox.ts` - Mobile notice references pages.dev ✅
+- `web/src/pages/*.ts` - All shareable/replay URLs use pages.dev ✅
+
+### Documentation Updates Required
+The following references `aicodebattle.com` and need updates:
+- `docs/plan/plan.md` - Multiple references to aicodebattle.com for shareable URLs
+- `docs/plan/plan.md` - References to b2.aicodebattle.com CDN (outdated - not used, current code uses R2 via Pages Functions at `web/functions/r2/[[path]].ts`)
+- `web/src/pages/docs.ts` - Example POST to api.aicodebattle.com (deferred API, doesn't exist)
+
+### Changes Made
+
+1. **Updated docs/plan/plan.md** - Replaced shareable URL examples from `aicodebattle.com` to `ai-code-battle.pages.dev`
+2. **Updated docs/plan/plan.md** - Added note that B2 CDN references are legacy, current implementation uses R2 via Pages Functions
+3. **Updated web/src/pages/docs.ts** - Marked api.aicodebattle.com examples as deferred/planned
+
+### Verification
+
+All user-facing absolute URLs emitted by web/src now resolve correctly:
+- ✅ https://ai-code-battle.pages.dev/ - Landing page
+- ✅ https://ai-code-battle.pages.dev/#/bot/{bot_id} - Bot profiles
+- ✅ https://ai-code-battle.pages.dev/#/watch/replay/{match_id} - Replay viewer
+- ✅ https://ai-code-battle.pages.dev/replay/{match_id}#turns=X-Y - Clip share URLs
+
+## Future Migration Path
+
+If/when `aicodebattle.com` is registered and configured:
+1. Add custom domain in Cloudflare Pages dashboard
+2. Search-and-replace `ai-code-battle.pages.dev` → `aicodebattle.com` in:
+   - docs/plan/plan.md
+   - web/src/og-tags.ts
+   - web/src/pages/clip-maker.ts
+   - web/src/pages/sandbox.ts
+3. Deploy and verify all redirects work
+4. Update this note with migration date
+
+## Rationale
+
+Using the Cloudflare Pages-provided domain (`ai-code-battle.pages.dev`) is the pragmatic choice:
+- **Zero cost** - No domain registration or DNS hosting fees
+- **Zero configuration** - Works out of the box with Pages deployment
+- **Zero delay** - No DNS propagation or SSL setup time
+- **Already implemented** - Web code already uses pages.dev correctly
+- **Reversible** - Can add custom domain later without breaking URLs
+
+The only downside is the less-branded domain name, which is acceptable for a pre-launch/prototype phase. When the project reaches production readiness and branding becomes important, the custom domain can be added with a simple find-replace and deploy.
+
+## References
+
+- Task: bf-5kk - Resolve canonical public domain
+- Cloudflare Pages custom domains: https://developers.cloudflare.com/pages/platform/custom-domains/
+- R2 Pages Functions: web/functions/r2/[[path]].ts