fix: remove --break-system-packages silent fallback in install.sh

[xiaolai]

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Bug

install.sh lines 70–76 silently fall back to pip install --break-system-packages when the standard pip install fails. This flag bypasses pip's protection for system-managed Python environments, which can corrupt system packages without the user's knowledge or consent.

On modern Linux distros (Debian 12+, Ubuntu 23.04+, Fedora 38+), system Python is externally managed. The --break-system-packages flag exists for explicit opt-in, not as a transparent fallback.

Fix

Replace the silent fallback with a clear failure message and virtual environment guidance, consistent with the script's existing error-surfacing pattern (e.g. the pip not found branch on line 78 already does this correctly).

Before

${PIP_CMD} install -q -r "${SKILL_DIR}/requirements.txt" 2>/dev/null \
    || { echo "  ⚠ Standard pip install failed, trying --break-system-packages..." >&2; \
         ${PIP_CMD} install --break-system-packages -q -r "${SKILL_DIR}/requirements.txt" 2>/dev/null; } \
    && echo "  ✓ Python dependencies installed" \
    || echo "  ⚠ pip install failed. ..."

After

if ${PIP_CMD} install -q -r "${SKILL_DIR}/requirements.txt" 2>/dev/null; then
    echo "  ✓ Python dependencies installed"
else
    echo "  ⚠ pip install failed. To install in a virtual environment:" >&2
    echo "    python3 -m venv ~/.venv && source ~/.venv/bin/activate" >&2
    echo "    pip install -r ${SKILL_DIR}/requirements.txt" >&2
    echo "  Or install globally (use with care): pip3 install -r ${SKILL_DIR}/requirements.txt" >&2
fi

Users who need the global install can still do it explicitly — the failure message tells them how.