fix(mcp): stop prepending bin to PATH, export LLM_BIN instead

[syf2211]

Summary

Stop prepending llm-functions/bin to PATH during MCP tool execution. Export LLM_BIN instead and spawn tools via absolute bin paths.

Motivation

When an agent tool script runs a system command by bare name (e.g. xclock), prepending bin/ to PATH causes the shell to resolve llm-functions/bin/xclock instead of the real system binary if an agent shares that name. This leads to recursive agent invocation or confusing failures (#225).

Changes

• mcp/server/index.js: remove PATH prepend; add resolveBinCommand() and spawn via absolute bin path; handle Windows .cmd shims
• scripts/run-tool.* / run-agent.*: export LLM_BIN for nested llm-functions invocations
• Document LLM_BIN in docs/environment-variables.md and mcp/server/README.md

Tests

argc link-web-search web_search_perplexity.sh
argc link-code-interpreter execute_js_code.js
argc test

• demo_sh and demo_js tests pass; output confirms LLM_BIN is exported
• demo_py.py test skipped by project tool list (pre-existing)

Notes

• Agent/tool scripts that need to call other llm-functions commands should use $LLM_BIN/<name> explicitly
• MCP server still invokes tools directly via absolute paths; no PATH mutation required

Fixes #225