feat(cli): add interactive REPL mode

[JacobFV]

Summary

• Run agi with no arguments to enter interactive mode — type tasks one after another in a REPL loop
• Running agi "some task" now returns to the input prompt on completion instead of exiting
• Events persist across tasks with separator dividers marking task boundaries
• Ctrl+C exits from both input and execution states

Changes

• src/cli.ts — goal positional is now optional ([goal] instead of <goal>)
• src/index.tsx — removed goal-required error gate
• src/components/PromptInput.tsx — new REPL input component
• src/components/EventDisplay.tsx — added separator event type for task boundaries
• src/hooks/useAgent.ts — driver cleanup/restart support, onFinishedRef to avoid stale closures
• src/app/App.tsx — phase state machine (input | executing) replacing one-shot lifecycle

Test plan

• agi "test task" — executes then shows input prompt (not exit)
• agi — shows interactive mode header and input prompt immediately
• Type a task in input prompt, verify it executes and returns to input
• Verify Ctrl+C exits from both input and execution states
• Verify space/q keybindings work during execution but not during input
• npm run typecheck passes
• npm run build passes

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