feat(sdk): add band-register-agent command bundling register-agent.sh

[nir-singher-band]

Summary

Share the agent-registration setup script across integration repos by shipping it inside the SDK as the band-register-agent console entry point — instead of vendoring it or piping curl | bash. Because the script now rides along with band-sdk, consumers pin it by pinning the SDK version, so it can't drift from the API version they install. Sharing collapses to: uv add band-sdk then band-register-agent.

This mirrors the existing band-acp / band-trigger entry-point pattern.

Changes

• Add band-register-agent console entry point (band.cli.register_agent:main) in pyproject.toml.
• Bundle register-agent.sh verbatim as package data ([tool.setuptools.package-data] "band.cli" = ["*.sh"]).
• Thin Python wrapper (src/band/cli/register_agent.py) resolves the script via importlib.resources and runs it with bash, mirroring its exit code (127 if bash is missing). stdout is passed through so the output stays eval-able.
• Optional positional api_key populates BAND_USER_API_KEY for the script; omitting it falls back to the env var. The bundled script's own argv-free key handling is unchanged.
• Add unit tests in tests/cli/test_register_agent.py.

Related Issues

None.

Testing

• Unit tests pass (uv run pytest tests/ --ignore=tests/integration/ --ignore=tests/e2e/) — 7 new tests added. Full suite green except pre-existing, unrelated failures: Codex-ACP @pytest.mark.e2e tests that spawn npx @zed-industries/codex-acp, and live-credential tests/integration/ tests.
• Pre-commit checks pass — ran the project checklist directly: ruff check, ruff format --check, and pyrefly check all clean.
• Integration tests pass (if applicable) — N/A (require live credentials).

Additional verification:

• Built the wheel and confirmed band/cli/register-agent.sh is bundled in the artifact (the real proof that "bundle as is" survives distribution).
• band-register-agent --help works; a no-key run exits 1 via the script's own BAND_USER_API_KEY guard.

Reviewer notes

⚠️ Intentional security tradeoff. The bundled script deliberately reads the key only from $BAND_USER_API_KEY (never argv) so it never appears in ps. The optional CLI arg is a convenience implemented in the wrapper — when used, the key is visible in the wrapper's process list. The env var stays the documented default, and --help spells out the tradeoff. The .sh itself is unchanged, so its stdin-only handling is preserved on the env path.

Checklist

• PR title follows Conventional Commits format
• Code follows project style guidelines
• Tests added/updated as needed
• Documentation updated as needed — follow-up: document band-register-agent in CLAUDE.md alongside band-acp / band-trigger.

✌️

[nir-singher-band]

Heads-up: a downstream consumer is waiting on this command.

band-ai/hermes-band-platform PR #5 switches the Band Hermes plugin's add-band install flow to use band-register-agent (band.cli.register_agent) instead of a bundled temporary helper. It's parked as a draft until this PR merges and band-sdk publishes a release that includes the command.

When this lands + publishes, please ping so we can: bump the plugin's band-sdk floor (its cap is currently <2.0.0), retarget PR #5 to dev, un-draft, and merge.