fix(release): merge concurrent main updates

[code-yeongyu]

Problem

Release preparation completed versioning, clean build, and the full serial test gate, then failed at git push origin main because PR #853 merged while the release was running. The release branch was non-fast-forward even though every release artifact was already verified.

Failed run: https://github.com/code-yeongyu/senpi/actions/runs/31680010156

Fix

• Fetch origin/main after the release tag and next-cycle commit are created.
• If remote main advanced, merge it with a normal two-parent commit before pushing main.
• Keep the release tag anchored to the already verified release commit.
• Add unit coverage for advanced, already-contained, dry-run, and call-order paths.

Verification

• Failing-first source proof confirms the old release flow had no pre-push synchronization.
• Focused tests: 4/4 passed.
• Root script suite: passed.
• npm run check: passed.
• Real two-clone Git simulation: concurrent main commit was preserved, next-cycle HEAD became a two-parent merge, and the release tag remained on the verified release commit.
• No v2026.8.13 tag or npm version exists.

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Summary by cubic

Synchronizes and merges concurrent origin/main updates during release before pushing to prevent non-fast-forward failures. Previously we pushed without syncing; now we fetch origin/main after creating the release tag and next-cycle commit and merge if needed, keeping the tag on the verified commit.

• Adds scripts/release-git.mjs and calls syncRemoteMainBeforePush in scripts/release.mjs after the next-cycle commit and before both pushes.
• If origin/main is not an ancestor, creates a two-parent merge on main; release artifacts and tag remain unchanged.
• Dry-run previews the fetch and conditional merge; unit tests cover advanced, already-contained, dry-run, and call order.

^{Written for commit 54bcd71. Summary will update on new commits.}