docs: document logical call telemetry and migration impact

Field counts come from inspect, not memory: record_llm_call takes 36
parameters, COLUMNS has 36 entries, the physical table has 37.

- README: capability table says 36 fields and names the three row kinds;
  new section covers reading call_stats, the five SQL migration items,
  the attribution query and the storage-side upgrade
- README/.env.example/ARCHITECTURE: error_body follows the summarize_body
  limit and the structured-exhaustion error carries its own bounded
  explanation, so neither is inside PGW_TELEMETRY_TEXT_CAP coverage
- ARCHITECTURE 7.8: the ten columns with per-column semantics, the I3/I4
  invariants, operation versus exc.operation, and the assembly gate
- CHANGELOG: unreleased section listing the four public changes and what
  downstream must do, in particular counting failures by event_kind and
  the assembly-time error for custom recorders
- schemas/llm-calls: the ten columns plus a three-row-kind section
- metrics/call-telemetry-coverage: 1.3.5 coverage contract, real live
  baselines left unfilled rather than stating a fake percentage

Validation record records the T4 evidence: mechanical migration red then
green, the four PG acceptance cases, the seven-item mutation matrix with
all seven killed and the copy restored to an identical digest, plus the
PYTHONPATH pitfall that made the first mutation round silently test the
original source.

Version numbers and release steps are deliberately untouched.

.env.example

@@ -113,6 +113,9 @@ PGW_TELEMETRY_BACKEND=none               # sqlite | postgres | none(必填)
 #                                               # 也要能拿原样的请求复现与重放;截断后这两件事都做不成,而既有下游正依赖这一行为。
 #                                               # 反面同样要看清: 不截断意味着客户合同、标书全文无限期留在 llm_calls 里,
 #                                               # 多租户下还混在同一张表。真在意留存面的部署应显式设一个上限,并配保留期与访问控制。
+#                                               # 1.3.5 补充: error_body(网关响应正文摘要)沿用库内 summarize_body 上限,
+#                                               # 结构化耗尽终态行的 error 说明自带有界限长(不含模型原始正文);
+#                                               # **两者都不在本键的覆盖面内**,估算留存面时要单独计。
 # PGW_PRICING_PATH=config/prices.json           # 可选: {"<model>": {"input_per_1m": x, "output_per_1m": y}};缺省 cost 恒 None
 #                                               # 可选第三档 "cached_input_per_1m": z —— 供应商 prompt cache 命中部分的单价;
 #                                               # 不填即命中部分也按 input 全额计(库不猜折扣率),cost 会偏高