Simplify hook output from JSON to plain text #17

Context

The hook currently outputs hookSpecificOutput JSON format:

{"hookSpecificOutput": {"hookEventName": "UserPromptSubmit", "additionalContext": "..."}}

This was originally filed as a bug fix for anthropics/claude-code#17550, which reported hookSpecificOutput JSON causing errors on the first message of a new session. However, testing confirms the JSON format works correctly for first messages in current Claude Code versions. The upstream issue is stale and pending autoclose.

Rationale

Switching to plain text is still a worthwhile cleanup:

• Simpler code: Removes JSON construction, string escaping, and the output_json() helper
• Fewer failure modes: No JSON parsing/escaping edge cases
• Equivalent functionality: Plain text stdout injects additionalContext the same way
• Defensive: Avoids any future regressions in JSON parsing

Steps

1. Refactor scripts/improve-prompt.py to output plain text instead of JSON
2. Update tests to validate plain text output format
3. Verify bypass prefixes still work with plain text output

References

• Upstream issue (stale): [BUG] UserPromptSubmit hook with hookSpecificOutput JSON shows error on first message of new session anthropics/claude-code#17550
• Claude Code hooks docs: plain text stdout is a supported alternative to JSON