revert(ai-http): drop resilient fetch/RetryAgent layer (#140)

The custom undici RetryAgent + aiFetch transport added for issue #140
did not actually heal mid-stream provider drops: undici's retry path is
a Range-based download-resume that SSE/chat-completions endpoints cannot
satisfy, so a reset after the first byte only swapped ECONNRESET for a
"server does not support the range header" error. Its only real effect
was reconnecting a poisoned keep-alive socket before the first byte, and
PR #141 on top of it turned the 60s headers timeout into deterministic
~61s failures (plus CONTENT_LENGTH_MISMATCH from retrying a POST body
after a timeout abort). The root cause is the z.ai coding endpoint, not
our transport.

Remove the whole layer and return all AI provider calls to Node's
default global fetch.

- delete integrations/ai/ai-http.ts and its spec
- ai.service.ts: drop the aiFetch import, the AI_BYPASS_RESILIENT_FETCH
  diagnostic toggle, and fetch:aiFetch from every chat/embedding/STT
  factory; raw STT call back to global fetch
- ai-chat.controller.ts: drop the stream-timing START log + startedAt
- ai-chat.service.ts: drop the first-chunk/FINISHED/ERROR timing logs
- .env.example: drop AI_BYPASS_RESILIENT_FETCH

Reverts: 1af5d34a, 7c308728, b7abb7ea, 35fc58ea, d6cd2754, 6efb8656.
Preserved (not part of the rollback): client-disconnect abort, title
generation in onFinish, partial-answer persistence, Safari SSE heartbeat.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

.env.example

@@ -128,11 +128,6 @@ MCP_DOCMOST_PASSWORD=
 # A slow/hung embeddings endpoint fails after this and the batch continues.
 # AI_EMBEDDING_TIMEOUT_MS=120000
 
-# Diagnostic: bypass the resilient outbound HTTP layer (custom undici RetryAgent)
-# for the CHAT model, using the default global fetch instead. Use only to isolate
-# a streaming/transport issue; leave unset in normal operation.
-# AI_BYPASS_RESILIENT_FETCH=true
-
 # --- Anonymous public-share AI assistant ---
 # Opt-in per workspace (AI settings -> "public share assistant"; off by default).
 # When enabled, anonymous visitors of a published share can ask an AI about that