fix: conflict merge data loss, attachment link preservation, diff noise, dry-run readability, duplicate ID detection

[rgonek]

Summary

This PR fixes six bugs and adds two quality-of-life improvements discovered during the Confluence sync CLI implementation review.

Changes

Bug Fixes

• Fix workspace restore before auto pull-merge (4505a67): cmd/push.go was invoking runPull while the push stash was not yet restored and was zeroing stashRef, risking loss of local workspace state during --on-conflict=pull-merge. The stash is now restored before invoking pull.

• Preserve attachment links through conversion (eeb542b): Local markdown file links to attachments were being rewritten to image syntax (![]()) during conversion prep, causing non-image attachments to be lost or rendered incorrectly. Fixed by using pandoc media-inline spans to preserve file attachment links.

• Reduce noisy markdown escaping on pull (2114708): The forward converter was over-escaping parentheses in markdown output (e.g. in links), creating unnecessary noise in pulled files.

• Fix media type inference regression (93afbeb): resolveForwardMediaType in hooks.go defaulted to "file" when MediaType was empty in an ADF node, even for images. Now infers the correct type from file extension (.png, .jpg, .gif, .webp, etc.) when type is absent.

Quality-of-Life Improvements

• Strip read-only metadata before diffing (524a0dc): conf diff was comparing raw local markdown (including updated_by, updated_at, created_by, created_at frontmatter fields) against remote snapshots that don't carry those fields, producing false diffs for unchanged pages. These fields are now stripped before comparison.

• Show Markdown preview in dry-run output (77fcc77): CreatePage and UpdatePage in dry_run_remote.go were dumping raw ADF JSON, which is hard to read. Dry-run output now shows a rendered Markdown preview via converter.Forward.

• Detect duplicate page IDs across files (4ff1533): No cross-file check for duplicate Confluence page IDs existed. Added detectDuplicatePageIDs in the validate flow, called after BuildPageIndexWithPending, to catch duplicate id values before any push.

Testing

• Added/updated unit and integration tests for all changed invariants.
• Full test suite passing: go test ./cmd/... ./internal/...
• go vet ./cmd/... ./internal/... clean.

Note: The pre-existing untracked dumper.go in the repo root (which imports a non-existent package) breaks go vet ./... and go test ./... at the root package level. This is intentionally left untouched as it predates this branch.