refactor(sse): consolidate client SSE readers behind a single typed primitive

Replace four hand-rolled client SSE decode loops with two layered
primitives in lib/core/utils/sse.ts:

- readSSELines: the single byte-stream decode engine. Splits on \n,
  strips trailing \r, tolerates data: with/without a leading space,
  skips the [DONE] sentinel, honors an AbortSignal before each chunk and
  between events, and releases the reader lock only when it acquired it.
- readSSEEvents<T>: a thin JSON layer that parses each payload and routes
  unparseable lines to onParseError (default: skip).

An SSESource union accepts a Response, a ReadableStream, or an
already-acquired reader so callers that must stash the reader for
external cancellation keep ownership of the lock.

Migrates use-execution-stream, chat use-chat-streaming, home use-chat
(via readSSELines for schema-validated decode), and the workflow chat
panel. Legacy server/wand exports (encodeSSE, SSE_HEADERS,
readSSEStream) are untouched. Behavior is preserved across abort, RAF
batching, TTS, [DONE], delimiter tolerance, and reader-lock ownership.

Tests in sse.test.ts pin the prior behavior: \n and \n\n framing,
mid-chunk splits, [DONE], data: with/without leading space, \r\n
stripping, sync/async early-stop, pre-aborted and mid-stream abort,
lock release/non-release per source, lock release on a throwing
handler, and Response/stream/reader sources.

Author: waleedlatif1