provider errors give no diagnostic detail, no turn resume, no duplicate-terminal detection

[Svector-anu]

issue 1: provider errors give no diagnostic detail

session events of type "error" only ever store the top-level message
string, e.g.:

{"type":"error","payload":{"message":"provider error: provider returned error"}}

no http status code, no response body, no upstream error cause is
persisted anywhere in events.jsonl. there is no --verbose/--debug flag
or env var to capture more detail either. when a provider call fails,
there is no way to determine why from the cli or its logs.

repro: session zero_20260713163241_1783960361665303000_1, events
#361, #586, #632, #646, #669, #689 (2026-07-14, 06:12–08:01 utc),
provider=gitlawb-opengateway, model=tencent/hy3. all 6 have identical
generic messages with zero additional context.

issue 2: no resume after a provider error mid-turn

after a provider error interrupts a turn, the next turn does not
resume from where it left off — it re-reads previously-read files
from scratch to rebuild context. work in progress at the time of the
error is lost, not just the single failed request.

issue 3: no visibility into duplicate background terminals across sessions

three separate bun run dev background terminals were left running
concurrently from three different zero sessions, started 06:33, 08:18,
and 10:54 utc on the same date, same command, same working directory.
nothing in the cli flagged that an identical background terminal was
already running before starting another.