--available-tools with an empty value exposes all tools instead of none #5306

Describe the bug

--available-tools is documented to disable every tool not listed, but an empty value (--available-tools= or --available-tools '') sends all 21 built-in tools, the same as omitting the flag. There's no documented way to run copilot -p with no tools.

The workaround in the last row works, but it relies on unknown tool names being silently ignored.

Affected version

GitHub Copilot CLI 1.0.105

Steps to reproduce the behavior

1. In an empty directory, run:

   copilot -p "Reply with the word ok." -s --model claude-haiku-5.5 \
     --available-tools= --disable-builtin-mcps --no-custom-instructions --no-ask-user
   

2. Read the tool count from the newest session's events:

   EVENTS=$(ls -t ~/.copilot/session-state/*/events.jsonl | head -1)
   jq -rn 'first(inputs | select(.type == "session.usage_checkpoint"))
     | .data.promptCacheBreakState[0].models[]
     | "\(.model): tool_count=\(.tool_count) tool_tokens=\(.tool_tokens)"' "$EVENTS"
   

   Output: claude-haiku-5.5: tool_count=21 tool_tokens=10439
3. Repeat steps 1 and 2 with --available-tools __none__.

   copilot -p "Reply with the word ok." -s --model claude-haiku-5.5 \
     --available-tools='__none__' --disable-builtin-mcps --no-custom-instructions --no-ask-user
   

   Output: claude-haiku-5.5: tool_count=0 tool_tokens=0

Expected behavior

An empty --available-tools exposes no tools, or there's a documented way to disable all tools and an empty value is rejected with an error.

Additional context

For text-only copilot -p calls in CI, the unwanted tool definitions add about 10,000 tokens per call. Related: #4596 (system prompt still references removed tools), #407 (list available tools).