feat: add buildStreamingCommand + migrate log/list follow to generator

Add streaming command infrastructure and convert the log/list --follow
path from a callback/timer-based polling loop to an async generator
that yields typed LogStreamChunk values.

New infrastructure (src/lib/streaming-command.ts):
- StreamingOutputConfig<TChunk>: human + jsonLine formatters
- drainStreamingOutput(): iterates generator, writes to stdout
  - Human mode: passes through pre-rendered text chunks
  - JSON mode: JSONL with array expansion (one line per log)
- buildStreamingCommand(): full command builder for purely streaming
  commands (e.g., auth/login). Generator receives StreamingContext
  with banner(), cwd, env, setContext, json — no stdout/stderr.
- abortableSleep(): resolves on AbortSignal for clean shutdown

log/list follow-mode migration:
- executeFollowMode (callback+timer) → generateFollowLogs (async gen)
- FollowConfig (stdout/stderr) → FollowGeneratorConfig (onDiagnostic)
- writeLogs helper eliminated — rendering done inside generator via
  yieldBatch() sub-generator and renderLogRows() pure function
- SIGINT handling: AbortController + abortableSleep replaces
  setTimeout/clearTimeout with stopped flag
- Banner extracted to writeFollowBanner() helper

Non-follow paths unchanged — still return CommandOutput<LogListResult>
with JSON array output.

Author: BYK