feat: wire SSE core into VS Code response panel + directives (#46)

[rwrife]

Closes #46 (partially — first shippable slice).

What changed

• New: src/core/sse/directives.ts — pure mapping from .http directives (@sse-until, @sse-max-events, @sse-max-duration-ms, @sse-idle-ms) to a validated SseTransportUserOptions object, plus an isSseResponse helper. Zod-validated. Bad numeric values are surfaced as diagnostics, not silently dropped.
• New: renderSseResponse in src/extension/responseView.ts — streaming-mode response panel that shows a scrolling event list with type / id / elapsed / timestamp, pretty-prints JSON data, and toggles between a "● streaming" and "■ " pill.
• Wired: runRequest now detects text/event-stream on the undici response and hands off to a new streamSseResponse helper that drives the existing pure runSseTransport over the decoded body, refreshing the webview on every event and surfacing @sse-until compile/runtime errors as a warning banner.
• Tests: 9 new unit tests covering directive parsing (happy path, empty predicate, non-integer caps, coexistence with unrelated directives) and the content-type sniffer. Total suite: 489 tests, all green.
• README: documented the new directive matrix and clarified what still lands in follow-ups.

What's left for #46

• Reconnect + Last-Event-ID retry loop (transport already tracks state; needs a socket-owning wrapper).
• Explicit "Stop stream" button in the webview (currently: close the panel or hit a guardrail).
• On-disk .sse.jsonl transcript command (formatSseTranscriptLine already exists in core).
• Integration test against a local SSE server via @vscode/test-electron.

Small, focused slice — no drive-by refactors, no changes to the existing HTTP path when the response isn't SSE.