docs(favorites): catch up on linked SQL folders v1.1

[datlechin]

Summary

Follow-up to #999 (Linked SQL Folders). The original PR updated docs/features/sql-favorites.mdx for v1 but left a few gaps:

• sql-favorites.mdx claimed linked folders are global only. Code already supports per-connection scope.
• v1.1 features were missing from the docs entirely: metadata dialog, live "modified on disk" banner, side-by-side diff sheet, drag-and-drop, non-UTF-8 encoding handling, BOM handling.
• Three other pages had related gaps: autocomplete.mdx never mentioned that linked-file @keyword participates in completion, icloud-sync.mdx didn't say folder paths are per-Mac, sql-editor.mdx SQL Files section had no coverage of the conflict diff or the modification banner.

Changes

• docs/features/sql-favorites.mdx — rewrote the Linked SQL Folders section. Fixed scope claim. Added per-connection vs. global wording. Documented metadata dialog, live banner, diff sheet, drag-and-drop, encoding handling. Tightened prose throughout.
• docs/features/autocomplete.mdx — new "Favorite Keywords" subsection in Completion Types with link back to sql-favorites.
• docs/features/icloud-sync.mdx — added "Linked SQL Folders" row to the What syncs table, marked not-synced with rationale.
• docs/features/sql-editor.mdx — extended SQL Files with "External modifications" subsection (banner + diff sheet + the three actions) and a "Linked folders" pointer.

Style notes

Followed CLAUDE.md writing rules:

• No em dashes anywhere (grep '—' returns zero matches across all four files).
• No banned filler words (seamless, robust, comprehensive, etc.) — verified with grep -inE.
• Concrete: file paths, key names, action labels, behavior numbers (~1s FSEvents propagation, etc.).

Test plan

• mintlify dev (or whatever local docs preview) renders all four pages without broken links or syntax errors.
• Cross-link [Linked SQL Folders](/features/sql-favorites#linked-sql-folders) resolves correctly from sql-editor.mdx.
• Cross-link [SQL Favorites](/features/sql-favorites) resolves from autocomplete.mdx.
• Read each updated section at human pace and check the prose doesn't sound like AI filler.

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