docs: scaffold agent instruction tree

Add AGENTS.md, CLAUDE.md, and the agents-docs/ tree (workflow, lessons,
engineering standards, context map, ADR seed, feature template) plus a
domain-bearing CONTEXT.md for each of the six subdomains: toju-app,
electron, server, e2e, website, docs-site.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

agents-docs/adr/0001-record-architectural-decisions.md

@@ -0,0 +1,13 @@
+# ADR-0001: Record Architectural Decisions
+
+## Status
+Accepted
+
+## Context
+We need a lightweight way to record architectural decisions so that future agents and engineers can understand *why* the system looks the way it does, not just *what* it does. Without ADRs, decisions live in PR descriptions, chat logs, or nowhere — and get re-litigated on every refactor.
+
+## Decision
+We use Architecture Decision Records (ADRs) in the Nygard short form. Each ADR lives at `agents-docs/adr/NNNN-slug.md` with a 4-digit zero-padded number, monotonically increasing. The minimum content is a title plus 1–3 sentences each for Context, Decision, and Rationale. Add `Status`, `Considered Options`, or `Consequences` only when they genuinely help.
+
+## Rationale
+Nygard short form is the lowest-friction format that still captures the *why*. Heavier templates (MADR, full IEEE 1471) routinely don't get written — the bar to start one is too high. ADRs are append-only: a superseded decision gets a new ADR with a `Supersedes ADR-NNNN` note while the old one stays in place. The 3-criteria gate (hard to reverse, surprising without context, genuine trade-offs) keeps the directory from filling with trivia. See `agents-docs/AGENTS_ADRS.md` for the full contract.