fix: conditionally include tools when OpenClaw is configured (fixes #12)

[tda1017]

Root Cause

• The Gemini setup path declared tool metadata unconditionally even when OpenClaw was not configured.
• The default assistant prompt remained execute-heavy, nudging tool invocation in a no-tool runtime.

Changes

• Added conditional tool declaration so tools is only included when OpenClaw is configured.
• Added a no-tools prompt fallback when OpenClaw is unavailable while preserving user-customized prompts.
• Added fast-fail routing in the tool router so unsupported tool calls fail immediately with explicit errors.

Testing & CI Evidence

• Verified fork CI run 22452316884 completed successfully.
• Both jobs are green: android-build and ios-build.

Testing Limitations

• Physical-device runtime execution was not performed in this environment.
• Validation here relies on CI and static/runtime-path checks available in this workspace.