ci: add coordinated main + preview release workflow (#995)

* ci: add coordinated main + preview release workflow

Adds a single workflow_dispatch that releases both branches together,
ensuring they stay in sync on npm.

* fix: address review — bump script compat, pre-publish verification, drop unused artifacts

- Preview bump now uses `prerelease --prerelease-tag preview` which the
  bump-version.ts script actually accepts
- Added verify-merges job that checks both main and preview have the
  expected versions before either publish runs (prevents drift)
- Both publish jobs now depend on verify-merges instead of each other,
  so neither publishes unless both PRs are confirmed merged
- Removed upload-artifact steps from test jobs since publish jobs
  rebuild from source post-merge

* fix: auto-rebase preview onto main in preflight step

Instead of failing when preview isn't rebased, the workflow now
rebases automatically. If there are conflicts, it aborts and
directs the user to resolve manually.

* ci: add sync-preview workflow, simplify release preflight

- New sync-preview.yml: runs on every push to main, auto-rebases
  preview onto main. Silently skips on conflicts (no failure).
- Release workflow preflight reverted to a simple check — relies
  on sync-preview having already done the rebase.

* fix: use merge instead of rebase for preview sync

Rebase overwrites preview-specific values (package version, tests).
Merge preserves preview's divergent files and only conflicts when
both branches touch the same lines.

* fix: concurrency control, conflict notifications, CDK tag TODO

- Add concurrency group to sync-preview to prevent parallel race
- On merge conflict, auto-create a GitHub issue (deduplicated) instead
  of silently skipping
- Add TODO comment for CDK preview dist-tag in prepare-preview

* fix: create PR with conflict markers instead of issue on merge conflict

On conflict, sync-preview now:
- Creates a branch with the merge conflict markers committed
- Opens a PR targeting preview with resolution instructions
- Tags the original commit author for visibility
- Deduplicates (skips if a sync PR is already open)

Author: notgitika