Update Plan Documents to Use Xref Syntax

Summary

Plan documents currently use plain-text references like Ref: SPEC-008 (Unbuilt Runes) and Dependencies: WORK-015. Replace these with {% ref %} tags so cross-references are machine-readable and will resolve to links once WORK-020 (entity registration) is complete.

Scope

All files under plan/work/, plan/spec/, and plan/decision/ that contain cross-references to other plan documents.

Examples

Before:

> Ref: SPEC-008 (Unbuilt Runes) — Package: `@refrakt-md/media`

After:

> Ref: {% xref "SPEC-008" /%} — Package: `@refrakt-md/media`

Before:

- Requires track and playlist runes (WORK-015) to exist as child content

After:

- Requires track and playlist runes ({% xref "WORK-015" /%}) to exist as child content

Acceptance Criteria

All SPEC-###, WORK-###, ADR-### references in plan documents use {% xref %} syntax
Blockquote Ref: lines use xref for each referenced ID
Dependency mentions in prose and lists use xref
No plain-text ID references remain (except inside attribute values like id="WORK-004")

Dependencies

WORK-026 (ref alias — so migrated files can use the shorter {% ref %} syntax)