docs: Magpie-style architecture and subsystem docs #39

FlyBot has coding rules (AGENTS.md, docs/code-standards.md, LESSONS.md, CONTRIBUTING.md) but no architecture docs like yetone/magpie's docs/subsystems/, integrating.md, reference.md. OpenCode/Codex lack module boundaries.

Scope (doc-only)

• docs/architecture.md: layers (pristine upstream ds-bot/ core vs FlyBot plugins vs root branding, per Keep ds-bot/ identical to upstream; FlyBot branding only at the root and in the plugins #33/Decouple the social and tree plugins from ds-bot so the core can track upstream #34), event/data flow.
• docs/subsystems/{README,ds-bot-core,plugin-runtime,tree,social,install}.md: owns / does not own / key files / entry points / invariants / tests / common mistakes.
• docs/integrating.md: writing a FlyBot plugin.
• docs/reference.md: commands and config keys found in code.
• AGENTS.md + code-standards.md: read the subsystem page before editing; update it in the same PR when boundaries change.

Acceptance

• Every claim matches actual files (no invented APIs); Codex PASS.
• Required check green.

Workflow: OpenCode writes, Codex reviews.