docs: extract subsystem guides; keep AGENTS.md to whole-repo facts

Move subsystem mechanics (migrations, thesis gate, redaction, ingest,
email, packaging) out of AGENTS.md into docs/guides/<topic>.md, each
scoped by paths: frontmatter and symlinked from .claude/rules/ so Claude
Code lazy-loads them. AGENTS.md keeps whole-repo facts and universal
guardrails plus a one-line index per guide. Fix the inaccurate
".claude/ is gitignored" note — it is tracked.

docs/guides/email.md

@@ -0,0 +1,23 @@
+---
+paths:
+  - backend/email_integration/**
+---
+
+# Email capture & drafts (Gmail)
+
+Read this before editing Gmail capture or draft creation.
+
+## What it does
+
+- `backend/email_integration/` captures Gmail via **domain-wide delegation** (`credentials.py`, `matcher.py`, `parser.py`, `db.py`, `sync.py`, `scheduler.py`, `routes.py`) and creates Tier-B in-thread drafts (`compose.py`). It has its own `migrations/`.
+- Captured email becomes CRM activity through a **propose → approve** flow — nothing lands on a contact record until a human approves the proposal.
+
+## Hard rule
+
+- **Agents draft; humans send.** Never let an agent send email, post, or contact an LP autonomously. Tier-B `compose.py` only *creates* a Gmail draft for human review.
+
+## Known gap
+
+- Tier-B drafts currently reply to the **LP only**; reply-all is the next change (see AGENTS.md → Current state).
+
+See also `docs/gmail-enablement-runbook.md`.