fix(tabs): advertise send_to_tab/read_tab in the agent system prompt

Granted tab-messaging tools were registered in the API tool payload but
buildSystemPrompt built its 'You have access to the following tools' list
by filtering toolNames through TOOL_DESCRIPTIONS, which had no entries for
send_to_tab/read_tab. The model was therefore told it lacked those tools
and refused to use them even when explicitly granted.

Add the two missing TOOL_DESCRIPTIONS entries so the capability list
matches the granted toolset. Add regression tests that capture the
constructed Agent's systemPrompt and assert the tab-messaging tools are
listed when granted (and omitted when not), locking the prompt list to
the schema list so they can't drift again.

0 files changed, 0 insertions, 0 deletions