fix: conditional requirements install and cross-platform venv activation

[saurabhhhcodes]

Closes #143

Problem

1. Dockerfile unconditionally ran COPY ${EXAMPLE}/requirements.txt — build crashes for examples without a requirements.txt (web3, mcp-agents, gemini-quickstart).
2. setup.sh hardcoded source .venv/bin/activate — fails on Windows Git Bash/MSYS2, where venv creates .venv/Scripts/activate.

Changes

Dockerfile

• Copy ${EXAMPLE}/ first, then install requirements only when the file exists:

RUN if [ -f requirements.txt ]; then \
        pip install --no-cache-dir --upgrade pip && \
        pip install --no-cache-dir -r requirements.txt; \
    else \
        echo "No requirements.txt found — skipping dependency install."; \
    fi

setup.sh

• Activate the venv from whichever path exists:

if [[ -f ".venv/bin/activate" ]]; then
    source .venv/bin/activate
elif [[ -f ".venv/Scripts/activate" ]]; then
    source .venv/Scripts/activate
else
    echo "Error: virtual environment activation script not found in .venv." >&2
    echo "Try deleting .venv and re-running this script." >&2
    exit 1
fi

Verification

• bash -n setup.sh — syntax OK
• Requirement-present / requirement-absent branches exercised in shell (skip message vs install path)
• Docker daemon not available locally; the RUN logic is plain POSIX if/else validated standalone