feat: add a generic external command handoff

[jayzeng]

Why

The strict MIT Core boundary intentionally removed product runtime, account, entitlement and host implementation code. That also removed the old command-dispatch path used by separately distributed functionality.

This restores only a product-neutral invocation seam. Core remains unaware of manifests, accounts, services, plans, capabilities or any separately distributed implementation.

What

• route the npm agent-memory bin through a small launcher
• keep all current Core commands in-process and unchanged
• for a safe unknown command name, look only for the fixed user-local ~/.agent-memory/bin/agent-memory-extension companion (.exe on Windows)
• invoke the companion directly without a shell and forward the original argv
• fall back to the existing Core CLI unknown-command behavior when no companion is installed
• compile the launcher for the standalone Bun/Homebrew binary too
• add deterministic resolver/command-name tests
• strengthen check:public-boundary with an explicit src/*.ts allowlist in addition to the existing lexical scan

Boundary properties

The public launcher contains no network access, remote origins, bundle loading, account state, authorization policy, licensing logic, host API, automatic capture implementation, or knowledge of separately distributed command names. Adding another public source module now requires an explicit boundary-check update.

Validation

CI should run the existing build/lint/unit/CLI/eval gates plus the new external-command test. The compiled launcher path should be exercised before merge because build:cli now compiles src/launcher.ts rather than src/cli.ts.

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