Add `mj` subcommand to launch `brokkai/mjolnir` as a pass-through

[foundev]

Summary

Add a new brokk mj subcommand that mirrors the existing acp and mcp launchers, but installs/resolves brokkai/mjolnir and runs it as a pass-through subprocess.

Why

The repository already has two patterns for launching bundled native servers:

• brokk acp launches Anvil over stdio
• brokk mcp launches bifrost over stdio

Both already support resolving a binary, forwarding passthrough arguments, and executing the binary in place.

Relevant existing evidence:

• README.md documents brokk acp and brokk mcp as stdio server entrypoints and notes that unknown mcp arguments are passed through to bifrost.
• brokk_code/anvil_launcher.py resolves/downloads Anvil and forwards passthrough args before exec/subprocess launch.
• brokk_code/bifrost_launcher.py resolves bifrost and forwards passthrough args before exec/subprocess launch.
• The current CLI structure in brokk_code/__main__.py already has separate runtime argument groups for ACP/MCP-style commands, which suggests mj could follow the same pattern cleanly.

Proposed behavior

• Add a new mj subcommand to the CLI.
• Resolve brokkai/mjolnir the same way the other native launchers resolve their binaries:
  • support an explicit binary override
  • support $PATH lookup
  • support cached/downloaded release resolution if needed
• Run the resolved binary as a pass-through process, forwarding unknown/extra CLI args to mjolnir.
• Keep the execution model consistent with existing launcher commands so stdio and exit codes behave predictably.

Acceptance criteria

• brokk mj --help is available.
• brokk mj ... launches mjolnir successfully.
• Extra args are forwarded to mjolnir unchanged.
• Behavior matches the existing style of acp/mcp launchers as closely as possible.
• Documentation is updated in README.md with the new command and resolution order.

Notes

If this command should also download a pinned release like Anvil/bifrost, please document the expected release asset naming and versioning scheme for brokkai/mjolnir.

[foundev]

Issue Analysis

The repository currently has no mj subcommand and no references to mjolnir. Implementing the feature requires adding a third native launcher that mirrors the existing acp and mcp patterns, plus CLI registration, dispatch, tests, and documentation.

What exists today

• brokk acp is wired in brokk_code/__main__.py and delegates to brokk_code/anvil_launcher.py. It uses _add_acp_runtime_args() to register --anvil-binary, --anvil-version, and passthrough Anvil flags. The launcher resolves the binary via override → $PATH (with version probe) → cached/downloaded release, then execs it.
• brokk mcp is wired in the same __main__.py module. It uses add_help=False on its subparser so parse_known_args() captures unknown arguments as passthrough args. It delegates to brokk_code/bifrost_launcher.py, which resolves the binary via brokk_code/rust_acp_install.py using the same override → $PATH (version-checked) → cache → download flow, then execs bifrost --root <ws> --server searchtools with passthrough args appended.
• Both commands reject Java executor flags (--jar, --executor-version) in _main_dispatch() before delegating.
• README.md documents both commands with explicit resolution order sections.

What is missing

1. No mj subcommand registration. _build_parser() in __main__.py needs a new subparsers.add_parser("mj", ...) with its own runtime argument group (e.g., --mjolnir-binary, --mjolnir-version, --workspace) and add_help=False so unknown args are forwarded.
2. No mjolnir launcher module. A new module (e.g., brokk_code/mjolnir_launcher.py) is needed that:
   • Defines a BUNDLED_MJOLNIR_VERSION constant.
   • Implements resolve_mjolnir_binary() with the same resolution order: explicit override → $PATH binary whose --version matches the bundled version → cached release binary → downloaded release from the brokkai/mjolnir GitHub release channel.
   • Implements run_mjolnir_server() that changes into the resolved workspace and execs the binary, forwarding passthrough args unchanged. On Windows it should use subprocess.run + sys.exit(returncode) instead of os.execvpe, matching Anvil and bifrost.
   • Uses the same caching, download lock, SHA256 verification, and archive extraction patterns found in anvil_launcher.py and rust_acp_install.py.
3. No CLI dispatch for mj. In _main_dispatch(), a new branch for args.command == "mj" must:
   • Reject --jar and --executor-version like acp and mcp do.
   • Convert the optional --mjolnir-binary string to a Path if provided.
   • Call the new launcher function with workspace_dir, binary_override, and passthrough_args=unknown.
4. main() passthrough logic needs updating. The check if unknown and args.command != "mcp" must also allow args.command != "mj" so unrecognized arguments are not rejected before they can be forwarded.
5. No tests. Following the existing test suite:
   • tests/test_bifrost_launcher.py shows the expected launcher test patterns: override resolution, $PATH preference, version mismatch fallback, cache hit skipping download, passthrough arg forwarding, and error handling for install failures.
   • tests/test_cli_modes.py shows the expected CLI dispatch tests: routing to the launcher, rejecting Java options, forwarding unknown args, and handling --help passthrough.
6. No documentation. README.md needs an mj section analogous to the MCP Server section, documenting the command, its resolution order, and any bundled version pinning.

Open design questions to resolve during implementation

• Release asset naming and versioning. The issue explicitly asks for documentation of the expected release asset naming for brokkai/mjolnir. The implementer should decide whether mjolnir releases follow the Anvil zip-with-SHA256 pattern or the bifrost tar.gz/zip pattern, and document the chosen scheme (including the asset filename template and version string format) in README.md.

Files to modify or create

• Create brokk_code/mjolnir_launcher.py
• Modify brokk_code/__main__.py (parser, dispatch, passthrough guard)
• Create tests/test_mjolnir_launcher.py
• Modify tests/test_cli_modes.py (add mj dispatch tests)
• Modify README.md

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Next steps: To fix this issue, run:

brokk issue solve --issue-number 9 --repo-owner brokkai --repo-name brokk-cli