feat: add first-class call_tool_chain

[Raezil]

Implements the first-class Client:call_tool_chain() workflow executor.

Includes:

• sequential tool execution
• previous and named-step state access
• structured step results and errors
• fail-fast by default
• on_error = "continue"
• opt-in reverse rollback via declared rollback tools
• regression tests wired into make test

The implementation is isolated in lua/utcp/chain.lua and exposed from lua/utcp/client.lua.

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Summary by cubic

Adds a first-class Client:call_tool_chain() workflow executor that runs steps sequentially with structured state, fail-fast by default, optional on_error = "continue", and opt-in reverse rollback.

^{Written for commit c7fa2f9. Summary will update on new commits.}