feat(sdk): support server-generated branch names

[necolas]

Why

The HTTP API can allocate a unique branch name when target_branch is absent, but the SDKs still require callers to choose a name. SDK callers also need access to the optional literal prefix supported by the API.

Scope

• Make the target branch optional in the TypeScript, Python, and Go SDKs.
• Add the optional target prefix to each SDK.
• Reject requests that set both target options.
• Add request-payload tests and examples for server-generated names.
• Update the Code Storage skill to match the public SDK contract.

Blast Radius

Existing calls with an explicit target branch keep their current request payload. The new behavior applies when a caller omits the target branch or supplies the new target prefix.

Verification

• pnpm --filter @pierre/storage build: passed.
• pnpm --filter @pierre/storage exec vitest --run: passed, 204 tests.
• ./venv/bin/pytest -q: passed, 184 tests.
• ./venv/bin/ruff check pierre_storage: passed.
• ./venv/bin/ruff format --check pierre_storage: passed.
• go test ./...: passed.
• python3 scripts/sync_versions.py --check: passed.
• git diff --check origin/main...HEAD: passed.

./venv/bin/mypy pierre_storage still reports the same three existing errors on main; this change does not add a type-check error.

[notion-workspace]

SDK: Server-generated branch names