Summary

• Implements CLIRuntimeBase abstract class in src/middleware/cli-runtime-base.ts (~200 lines)
• Implements AgentRuntime interface with an async generator execute() method
• Provides shared subprocess machinery: NDJSON parsing, watchdog timer, abort signal propagation, stderr capture, stdin prompt delivery for long prompts (>10K chars)
• Defines three abstract methods for concrete runtimes: buildArgs(), extractEvent(), buildEnv()
• Adds 16 unit tests covering all acceptance criteria in src/middleware/cli-runtime-base.test.ts

Closes #5

Test plan

• NDJSON parsing: valid lines, malformed lines, empty lines, null extractEvent results
• Abort signal: fires mid-execution, already-aborted signal
• Watchdog timer: kills on inactivity, resets on output
• Stdin prompt delivery: long prompts written to stdin, short prompts not written
• Subprocess spawning: command/args, working directory, environment merging
• Stderr capture: captured without producing error events
• Done event: includes duration, always emitted last
• pnpm check passes (format + typecheck + lint)

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