[#912 Slice 1] Extract editor-core.ts + consolidate preprocessMarkdown #919

Acceptance Criteria

• editor-core.ts exports baseExtensions, editableExtensions, loadLanguage, buildThemeExtension, buildSyntaxHighlightStyle, detectMode, and the externalUpdate annotation — all with their existing signatures
• baseExtensions(opts: { theme: Extension, language?: LanguageSupport | null }): Extension[] — signature preserved exactly
• editableExtensions and its co-located externalUpdate annotation both move to editor-core.ts (the annotation is referenced inside editableExtensions's update listener filter)
• detectMode(): "light" | "dark" is extracted (it reads the theme from CSS and is needed by any consumer of buildThemeExtension)
• editable-diff-view-core.ts imports from editor-core.ts — its own exports and behavior are unchanged
• markdown-utils.ts exports a single preprocessMarkdown implementation — the canonical version with newlines and trim: `$$\n${body.trim()}\n$$`
• markdown-utils.ts exports RENDERABLE_EXTENSIONS — the single source of truth for file type filtering: { markdown: [".md", ".markdown"], images: [".png", ".jpg", ".jpeg", ".gif", ".svg", ".webp", ".ico", ".bmp"], pdf: [".pdf"] } and a helper isRenderable(filename: string): boolean
• session-preview-tab.tsx and session-review-file-preview-v2.tsx both import preprocessMarkdown from the shared module
• The test in markdown_preview.test.ts imports from the shared module and its assertions are updated to match the canonical implementation (the test currently defines its own copy WITHOUT newlines; 4 assertions need updating to expect the \n wrapping)
• Files Changed tab behavior is completely unchanged (existing tests pass)

Testing Decisions

Run the existing test suite (pnpm --filter amicode test) — all tests must pass. Update markdown_preview.test.ts to: (1) delete its local preprocessMarkdown definition, (2) import from markdown-utils.ts, (3) update the 4 math-block assertions to expect the canonical $$\n...\n$$ format with trim. No new test files needed — this is a pure extraction. Add a unit test for isRenderable() covering each extension category and rejection of non-renderable extensions.

Key Decisions

Signatures preserved exactly. baseExtensions(opts: { theme: Extension, language?: LanguageSupport | null }), buildThemeExtension(mode: "light" | "dark"), loadLanguage(ext: string), detectMode(): "light" | "dark" — no wrappers, no adapters. The diff view's call sites do not change at all.

externalUpdate moves with editableExtensions. The annotation is referenced inside the update listener filter in editableExtensions. They are co-located in editor-core.ts and both re-exported.

preprocessMarkdown canonical version uses newlines + trim. The source implementation (`$$\n${body.trim()}\n$$`) is correct for KaTeX display math rendering. The test's version and the review file's copy (both no-newlines, no-trim) are updated to use the canonical import. The test assertions must change to match.

RENDERABLE_EXTENSIONS lives in markdown-utils.ts. This constant is consumed by Slices 2, 3, and 5. Defining it once here avoids duplication and gives future slices a single import.

Constraints & Invariants

• Zero behavior change to the Files Changed tab — this is a pure extraction
• No new dependencies added to session-ui
• All three preprocessMarkdown call sites use the single shared export after this slice
• The externalUpdate annotation is available from editor-core.ts for any future editor consumer

Prior Art

• editable-diff-view-core.ts — the extraction source (functions at lines ~77, ~105, ~220, ~245, ~527; externalUpdate at line ~46)
• session-preview-tab.tsx line 36, session-review-file-preview-v2.tsx line 39, markdown_preview.test.ts line 17 — the three preprocessMarkdown copies

Source

Part of #912