Add supervised R workers and structured handoffs

[TroyHernandez]

Long R computations need an enforceable deadline without delivering a delayed interrupt into the host's next operation. Sessions can now opt into run_r_mode = "worker", with one persistent callr worker per session. Timeouts interrupt that worker and report whether partial state survived; unresponsive workers are terminated.

skill_timeout now reaches ordinary and self-bounded tools. Supervised R calls also respect skill_timeout_max and an optional session$run_r_timeout_cap callback for changing host deadlines. Worker-owned handles and subagent return_name artifacts resolve in their owning process. Host checkpointing preserves nested helpers and shared environment references, and bounds checkpoint writes too.

Subagent queries may opt into report = TRUE for structured status, notes, findings, concerns, deviations, open questions, and feedback. Reports remain readable in the ordinary character reply and are also available as structured metadata. Timeout guidance advises inspecting state and improving, splitting, parallelizing independent work, or approximating the computation before retrying.

Compatibility: in-process execution remains the default; tool_run_r(code, envir) is unchanged; existing positional arguments and two-argument custom executors keep working. New report arguments are appended and default to false.

Verification: the complete offline installed suite passes 4,734 assertions; the final worker tests pass 60/60. Focused subagent, schema, observer, timeout, and positional compatibility tests pass. CI dependency installation preserves its built-in GitHub token through sudo to avoid anonymous download rate limits.