Stop fable's context from being spilled to a preview

[fcakyon]

#262 raised the truncation caps, which pushed fable's injected context from 22KB to 65KB. Past ~64KB the harness spills additionalContext to disk and hands the subagent a ~2KB preview, so the reviewer ended up with less than before. Codex never hit this because it already stages to a file.

	before	after
emitted payload	67,302 B	293 B
reviewer sees	2KB preview	whole conversation

• the agent is told to read a path now, so its marker line stays truthful
• one terminal emit, so a throw can no longer write two JSON objects
• contextFile is set after the write returns, so the path always resolves

-emit(`You are reviewing an in-progress Claude Code session. ...${recent}...`)
+writeFileSync(staged, `You are reviewing an in-progress Claude Code session. ...${recent}...`)
+emit(`Read ${staged} before answering. ...`)

Verified across 8 hook inputs (missing file, malformed stdin, null path, empty file, junk lines, directory as path): all exit 0, all emit exactly one object, none above 341 B.