fix(newsletter): render Prompt custom sections

[bokelley]

Summary

• render The Prompt custom sections in generated email HTML, plain text, and Slack output
• include custom sections and pasted content in published perspective markdown
• route scheduled perspective publishing through the canonical Prompt markdown builder while preserving non-blocking failure handling

Root cause

The admin editor persisted custom sections, but The Prompt's normal generated-content render paths did not consume them. Scheduled perspective publishing also used a separate legacy markdown builder, so it could diverge from test sends and admin-triggered sends.

Impact

Custom text blocks saved in the newsletter editor now appear in test sends, scheduled delivery, Slack, web previews, and published perspectives. Position 0 renders before generated sections and positive positions render after them, matching The Build's existing behavior.

Closes #5873.

Validation

• focused Prompt renderer, cover-image, and weekly-digest job tests
• npm run typecheck
• git diff --check
• independent code, test, and newsletter-workflow expert reviews

No changeset is required because this is server-side newsletter behavior only.

[aao-secretariat]

Clean fix. Custom sections were persisted by the editor but never consumed by the generated-content render paths — this closes that gap across all four surfaces and consolidates scheduled publishing onto the canonical Prompt markdown builder.

Things I checked

• Tracking-id stability (server/src/addie/templates/weekly-digest.ts:148-156): index is captured via .map((section, index) => ({ section, index })) before the placement .filter, so IDs stay stable and non-colliding across the before/after passes. Test asserts custom_1_1 and custom_2_1 — only passes if the index precedes the filter. Right shape.
• Placement logic is consistent across all four paths — email HTML, plain text, Slack, and perspective markdown all treat position 0 as before generated content and position >0 as after. Matches The Build's existing behavior.
• digest-publisher refactor (server/src/addie/services/digest-publisher.ts): buildFullMarkdown (74 lines) deleted, the single caller now takes a markdown param, and DigestContent is still load-bearing via extractTags/openingTake — no dead import, no orphaned caller. The scheduled path and test/admin sends can no longer diverge.
• Non-blocking failure handling preserved (server/src/addie/jobs/weekly-digest.ts:337): void (async () => { ... })().catch(...) keeps publishing off the critical path; import() and buildMarkdown throws are caught. Happy path is unchanged.
• No changeset, correctly. No file touches static/schemas/source/**, docs/reference/**, mintlify-docs/reference/**, or the schema-build pipeline. Nothing on the wire. MUST FIX #6 doesn't fire.

Minor nits (non-blocking)

1. position doc comment now overstates its role. server/src/newsletters/config.ts documents CustomSection.position as an insertion index, but the placement model is binary (0 = before, >0 = after) — sections at positions 1 and 5 render in array order, not position order. Intentional (matches The Build), but worth tightening the comment to "0 = before generated content, >0 = after."
2. Dynamic import intent. The import('../../newsletters/the-prompt/index.js') inside the IIFE at weekly-digest.ts:337 is presumably breaking a circular import. A one-line comment saying so keeps a future maintainer from "cleaning it up" into a static import and reintroducing the cycle.

code-reviewer: ready to push — no blockers, no majors. Note that the reviewer's environment blocked the PR-branch fetch, so the suite wasn't run locally; confirm CI is green before merge.

LGTM. Follow-ups noted below.