feat: pure SSE transport driver with reconnect + stop conditions (#46)

[rwrife]

Refs #46

Adds a pure, VS Code-free SSE transport driver on top of the existing frame parser and @sse-until gate. Complements #46 without pulling in the response-viewer UI in the same PR.

What changed

• src/core/sse/transport.ts — new module:
  • runSseTransport({ input, onEvent, ... }) drives an async iterable of decoded string chunks through SseParser, calling onEvent per dispatched event with { index, elapsedMs, reconnect } meta.
  • Stop conditions: AbortSignal, @sse-until match, maxEvents, maxDurationMs, idleMs — all enforced between events so a multi-frame chunk can't blast past a cap.
  • SseReconnectState tracks lastEventId and server-suggested retryMs across attempts.
  • reconnectHeaders(state) produces the Last-Event-ID header per spec (omitted when unset/empty).
  • clampRetryMs(...) bounds absurd server values into 100 ms – 30 s by default.
  • formatSseTranscriptLine(...) emits one JSON record per line for the .sse.jsonl transcript format.
  • SseTransportUserOptionsSchema (zod, .strict()) validates directive-sourced options before they touch the driver.
• src/core/sse/index.ts — re-exports.
• test/sse.transport.test.ts — 22 vitest cases covering happy paths, chunk-boundary framing, every stop condition (with injected clocks + AbortController), @sse-until errors, transcript formatting, header omission, retry clamping, and schema validation.
• README.md — Streaming responses (SSE) section with an OpenAI-style example and links to [feature] Server-Sent Events (SSE) request support #46 for the UI follow-up.

Acceptance criteria progress (#46)

• Pure SSE frame parser in src/core/sse/ with vitest coverage — already existed; kept intact.
• Zod schema for parsed events — SseEventSchema (parser) + SseTransportUserOptionsSchema (driver options).
• Response viewer streaming mode with live event list — follow-up PR (needs webview work).
• # @sse-until <expr> directive parsed + evaluated in sandboxed context — parsed generically today; now wired through the transport driver.
• Reconnect + Last-Event-ID behavior — implemented as pure state; integration test against a real HTTP server lands with the undici wiring PR.
• README section with an OpenAI-style streaming example.
• No plaintext auth material logged in transcripts — driver never writes; formatSseTranscriptLine only reflects event fields.

What's left

Follow-up PR: wire runSseTransport into the undici request path (text/event-stream detection, response-viewer streaming mode, Stop button, transcript save command, reconnect loop honoring clampRetryMs).

Checks

• npm run lint ✅
• npm run typecheck ✅
• npm run test:unit — 480 tests pass (was 458).