Add fallback logging to aikido-precommit version resolution

[Copilot]

Addresses review feedback requesting user visibility when version resolution falls back from "latest" to the hardcoded default.

Changes:

• Added warning messages to stderr when upstream version fetch fails (network/timeout) or version extraction fails
• Warnings specify the fallback version being used and the reason for fallback

Implementation:

# Two distinct failure modes now surface to users:
if upstream_script=$(curl -fsSL --connect-timeout 10 "$AIKIDO_UPSTREAM_SCRIPT" 2>/dev/null); then
    if [ -n "$extracted_version" ]; then
        echo "$extracted_version"
        return 0
    fi
    echo "Warning: Could not extract version from upstream script, using fallback version $FALLBACK_VERSION" >&2
else
    echo "Warning: Failed to fetch upstream version (network or connection timeout), using fallback version $FALLBACK_VERSION" >&2
fi

Users now understand when and why they're not getting the latest version when version: "latest" is specified.

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