[feature]: Document the LOOPFLOW_CLAUDE_BIN environment variable

[faisalishfaq2005]

What problem would this solve?

src/core/claude.ts lets users override the Claude binary path via the LOOPFLOW_CLAUDE_BIN environment variable — useful for non-standard installs and CI — but it's undocumented. Add it to the README (a short "Configuration" note or the CLI reference section).

Proposed solution

Good first issue for someone who wants to start by reading the code and improving the docs.

Alternatives considered

No response

Scope

• I've checked this isn't already on the roadmap in the README.
• This can't reasonably be expressed as a loop instead of engine code.