Create and assign Linear ticket before solving an issue

[codegen-sh]

Description

This PR implements the feature to create a Linear ticket before solving an issue, as requested in CG-11608.

Changes

1. Updated the system prompt in prompts.py to include the instruction to create a Linear ticket before solving an issue
2. Modified the create_codebase_agent and create_chat_agent functions to include Linear tools when available
3. Added proper error handling for when Linear client is not configured

How it works

When a user asks the agent to solve an issue, the agent will:

1. Create a Linear ticket to track the work
2. Assign the ticket to itself
3. Proceed with solving the issue

This ensures that all work is properly tracked in Linear, even when initiated directly from Slack.

Testing

The changes are conditional on Linear being configured, so they won't affect environments without Linear integration.

Closes CG-11608

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