[ENG-2098] Materialize Roam-origin markdown into Roam

[sid597]

Accepts Roam-origin shared nodes in the Roam materializer: the expected full content type is now derived from the node's platform (text/roam+markdown for Roam, text/obsidian+markdown for Obsidian), and Roam-origin full content gets its leading # title heading stripped on import when the first line matches the shared title exactly — otherwise the text is left untouched. The producer format stays unchanged; strip-on-import keeps the validated Roam→Obsidian path as is.

The platform gate in validateSharedNode and isImportableSharedNode in the discovery dialog are removed rather than extended: with both Platform enum values (Roam, Obsidian) now supported, both checks would be dead code by type.

Stacked on #1269 (ENG-1859).

Scope check

• Ran $scope-check against ENG-2098 and the final diff.
• Scope beyond Done When: None.

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[linear-code]

ENG-2098

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[graphite-app]

PR size/scope check

This PR is over our review-size guideline.

• Recommended: ~200 lines changed
• Acceptable limit: up to 400 lines when well-scoped/self-contained
• Preferred file count: fewer than 5 files

Please split this into smaller PRs unless there is a clear reason the changes need to land together.

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[devin-ai-integration]

✅ Devin Review: No Issues Found

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[sid597]

@codex review

[chatgpt-codex-connector]

Codex Review: Didn't find any major issues. What shall we delve into next?

Reviewed commit: 090f66c9ce

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