Add readiness_score_feedback as externally stored markdown field on WorkItems::AgentPlan

What does this MR do and why?

Adds readiness_score_feedback as an externally stored markdown field on WorkItems::AgentPlan, sharing the same object-storage blob as the existing content field.

Part of #617982 (closed) — MR 1 of 3 (model and persistence). MR 2 (GraphQL read) and MR 3 (GraphQL write) are stacked on this branch and target it; they will be retargeted to master once this merges.

What changes

• ee/app/models/work_items/agent_plan.rb: adds cache_markdown_field :readiness_score_feedback, storage: :external, issuable_reference_expansion_enabled: true beside the existing content line, and a length validation reusing CONTENT_LENGTH_MAX.
• ee/app/models/work_items/widgets/agent_plan.rb: delegates :readiness_score_feedback and :readiness_score_feedback_html through the widget, mirroring content and content_html. The _html delegation is what makes the cached rendering reachable from GraphQL; MR 2 exposes it as a markdown_field.
• ee/app/services/work_items/data_sync/widgets/agent_plan.rb: adds a second MarkdownContentRewriterService call for :readiness_score_feedback on work-item move. The service rewrites one field per call, so the new field needs its own call.

How it works (no migration needed)

cache_markdown_field :x, storage: :external registers both x and x_html as virtual ActiveRecord attributes via externally_stored_field. build_external_payload serializes all registered fields into one JSON blob whose filename is deterministic (#{work_item_id}.json). A second field therefore reuses the same blob and the existing file_store column. Existing blobs that predate this field read back as nil (the loader does payload[field.to_s], so a missing key is nil).

Risk note

The external storage mechanism may shift underneath this work (see #616229 (comment 3694614696) and !249644 (merged)). This MR should be coordinated with that work before landing.

Concurrent-write note

persist_external_payload does an unlocked read-merge-write. With two externally stored markdown fields, concurrent refresh_markdown_cache! calls touching different fields on the same record could clobber each other. This is only reachable from refresh_markdown_cache! and is a pre-existing concern for content + content_html; adding a second field makes it slightly more likely. Flagging for awareness — not a blocker for this MR.

References

• Issue: #617982 (closed)
• Precedent MRs: !249416 (merged) and !249498 (merged)

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