feat(runtime): add supervised run sessions

Summary

• Adds supervised target sessions through once runtime start/status/logs/stop, returning a session id while stdout, stderr, and status persist under .once/runtime/.
• Exposes the same session model to agents through opt-in MCP tools behind once mcp --allow-run: once_start_target, once_runtime_status, once_runtime_logs, and once_stop_runtime.
• Keeps run and toolchain knowledge out of generic Rust by leaving Apple output and path conventions in Starlark-declared actions and providers, and removes the previous Rust-side Apple fallbacks.
• Refreshes generated CLI and MCP reference docs, removes MCP cross-references from non-MCP CLI help, and removes internal planning pages from docs/.

Testing

• mise exec -- cargo fmt --all -- --check
• mise exec -- cargo clippy --workspace --all-targets -- -D warnings
• mise exec -- cargo test -p once-cli commands::runtime
• mise exec -- cargo test -p once-cli commands::mcp
• mise exec -- cargo build --release -p once-cli
• mise exec -- shellspec spec/runtime_spec.sh

Post-Deploy Monitoring & Validation

No additional operational monitoring required. This changes local CLI, MCP, graph metadata, fixtures, and docs only.

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