feat(sdk): support ephemeral merge previews

[necolas]

Summary

• Add optional source and target ephemeral namespace flags to previewMerge in TypeScript, Python, and Go.
• Send source_is_ephemeral and target_is_ephemeral to the existing merge preview endpoint.
• Preserve explicit false values and omit absent values.
• Update the three package READMEs, the Code Storage skill, and the release notes.

Stack

This PR is stacked on #47. Merge #47 first, then retarget this PR to main.

This PR does not include #50. It also does not change .version or the monorepo package catalog. A release PR owns those changes.

Compatibility gate

The TypeScript and Python changes are additive. The Go change adds exported fields to PreviewMergeOptions. It preserves documented keyed literals, but external unkeyed literals can stop compiling. The release owner must decide whether the SDK supports unkeyed option literals before publication.

Verification

• pnpm exec vitest --run: 218 tests passed.
• pnpm build: passed.
• ./venv/bin/pytest -v: 193 tests passed.
• ./venv/bin/ruff check pierre_storage: passed.
• ./venv/bin/ruff format --check pierre_storage: passed.
• ./venv/bin/mypy pierre_storage: passed.
• go test ./...: passed.
• gofmt -d *.go: no diff.
• moon :test: passed for all three SDK packages.
• git diff --check, the conflict marker check, and the secret-pattern scan passed.

[notion-workspace]

Support ephemeral merge previews in the SDKs

[fat]

lgtm!