docs: dedupe structure and stack descriptions (finding 32)

Why: AGENTS.md's Structure bullet restated apm-install mechanics already owned by
docs/spec/architecture.md:24 and README.md:55, and docs/VISION.md carried stack, framework and
deployment choices for a product that lives in a separate repo.

Implementation notes: AGENTS.md keeps two actionable one-liners plus pointers to the README
layout table and architecture.md, preserving the session rule that .claude/skills/ and
.claude/agents/ are install output and must not be edited. VISION.md's Phase 1 Architecture
block becomes a one-line scope statement; the "Mobile/desktop (Phase 3)" line is dropped as an
intra-file duplicate of the Phase 3 section.

Impact: no behaviour change. README.md and docs/spec/architecture.md are untouched -- the
finding's premise was inflated, and architecture.md had already been differentiated in a way it
documents in the file itself.

Refs: SIMPLIFICATION-AUDIT.md finding 32
Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01YR2CjVumUbEGWcMikcoXBD

AGENTS.md

@@ -6,8 +6,10 @@ This file carries only what applies to **every** session. Setup, prerequisites,
 
 ## Structure
 
-- `plugins/` — six installable plugin units, each an apm package (`apm.yml` + `.apm/`). Root `apm.yml` declares all six as `dependencies.apm`; `apm install` deploys them into `.claude/skills/` and `.claude/agents/`, both gitignored install output.
-- `providers/claude-code/` — Claude Code adapter, deployed to `~/.claude/` via `scripts/install.sh`.
+- `plugins/<name>/.apm/` is the only authoring source for plugin content. `.claude/skills/` and `.claude/agents/` are gitignored `apm install` output — never edit them.
+- `providers/claude-code/` is the Claude Code adapter, deployed to `~/.claude/` by `scripts/install.sh`.
+
+Repo layout table: `README.md`. Deployment mechanics and plugin boundaries: `docs/spec/architecture.md`.
 
 ## Prefer plugin skills over raw shell