feat(content-build): emit routing + secrecy lint + writer

tools/content_build/tests/fixtures/valid/world.md

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+# Test bible
+
+```yaml
+id: rule.disposition-ladder
+type: rule
+status: canon
+secrecy: 0
+related: []
+rungs: [hostile, cold, neutral, warm, trusted]
+body: >
+  The five-rung ladder.
+```
+
+```yaml
+id: town.testburg
+type: town
+status: canon
+secrecy: 0
+related: []
+body: >
+  A test town.
+```
+
+```yaml
+id: rumor.something
+type: rumor
+status: canon
+secrecy: 1
+related: [town.testburg]
+body: >
+  A rumor body.
+```
+
+```yaml
+id: secret.big-twist
+type: secret
+status: canon
+secrecy: 4
+related: [town.testburg]
+body: >
+  The twist body. Server-only.
+```
+
+```yaml
+id: npc.tess
+type: person
+status: canon
+secrecy: 0
+start_disposition: cold
+related: [town.testburg]
+body: >
+  Tess persona.
+knows:
+  - {fact: rumor.something, gate: neutral}
+  - {fact: secret.big-twist, gate: never}
+disposition_notes: >
+  Author notes.
+```