Correct stale placeholder path examples in the diagnostics guide

[pengfei-threemoonslab]

Found while implementing #543; deferred as a small documentation correction rather than mixing an unrelated historical example into identity recovery.

Evidence

docs/diagnostics.md under Human-owned placeholders still says collect_placeholders names a list item by its own text and shows agent.declared_purpose.CHANGE_ME. The implementation in cli/discovery/placeholders.py walks YAML nodes and emits agent.declared_purpose[0] with the source line. Existing tests/test_setup_control.py explicitly rejects the older path shape, including flow-style YAML.

An integration author copying the guide would match a path the current producer does not emit. The field ownership itself is correctly enforced; this is a stale explanation, not a newly observed routing bypass.

Proposed correction

Update the example and explanation against the actual helper output; check nearby diagnostics examples for the same obsolete list-path spelling. Preserve the existing schema and ownership model.

Acceptance

• The guide shows agent.declared_purpose[0] for both block and flow-style list placeholders.
• The prose describes YAML-node-derived field paths and line locations, consistent with the existing tests.

Related: #543. This documentation debt does not block its recovery implementation.