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.

.claude/rules/email.md

@@ -0,0 +1 @@
+../../docs/guides/email.md

.claude/rules/migrations.md

@@ -0,0 +1 @@
+../../docs/guides/migrations.md

.claude/rules/packaging.md

@@ -0,0 +1 @@
+../../docs/guides/packaging.md

.claude/rules/redaction.md

@@ -0,0 +1 @@
+../../docs/guides/redaction.md

.claude/rules/spark-ingest.md

@@ -0,0 +1 @@
+../../docs/guides/spark-ingest.md

.claude/rules/thesis.md

@@ -0,0 +1 @@
+../../docs/guides/thesis.md