#1572 docs(pm): track extension custom-field projection report

Tracks #1571 in pm-gh1571 so its reproduction and implementation criteria are available in the canonical project context. The report arrived after #1570 merged.

This records an open, unclaimed issue under the extension platform epic, with the original source, expected and reported behavior, acceptance criteria, and links to adjacent completed work. No runtime change or independently reproduced fix is claimed.

Validation: build; tracker context quality (zero blocking findings, existing graph-degree advisory); graph composition; record integrity; external pm-changelog generation check; whitespace check. The open item adds no changelog release entry.

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Summary by cubic

Intakes GitHub #1571 as an open, unclaimed issue in the PM tracker under the extension platform epic, recording the reproduction, acceptance criteria, and surrounding context without claiming any fix.

• Records the distinct refusal paths: SDK get raises its fields-contract error, while CLI get/list/search emit the structured unknown_field_projection refusal.
• Adds the issue record and its append-only history with source, observed/expected behavior, validation results, and links to adjacent completed work.
• No runtime change or changelog entry, since the issue is open and unreproduced independently.

Summary by Sourcery

Track the open extension-platform issue and its surrounding project context without introducing runtime or release changes.

Enhancements:

• Record GitHub issue #1571 in the PM tracker with its source, reported and expected behavior, acceptance criteria, validation context, and related work.

Chores:

• Add append-only history for the newly tracked open issue without claiming an independent reproduction or fix.