action: center provider-error log excerpt on the matched line (closes #703)

the `» provider error detected (...)` excerpt was `chunk.substring(0, 500)`
— the head of whatever stderr buffer node delivered. on big writes that's
the front of an mcp tool-schema dump, not the matched error text. label
was correct (regex.test on the whole chunk), excerpt was misleading.

introduce findProviderErrorMatch(text) that returns { label, excerpt }
where excerpt is a windowed slice centered on the regex match index:
the matched line plus 1 line before and 2 lines after, hard-capped at
600 bytes. detectProviderError stays as a thin wrapper for label-only
callers. both opencode and claude harnesses log match.excerpt instead
of chunk.substring(0, 500).

regression tests cover the multi-line buffer case, surrounding-line
context, byte-cap fallback to matched-line-only, and head truncation
of a single oversize line.

agents/opencode.ts

@@ -20,7 +20,7 @@ import { BEDROCK_MODEL_ID_ENV, modelAliases } from "../models.ts";
 import { getIdleMs, markActivity } from "../utils/activity.ts";
 import { formatJsonValue, log } from "../utils/cli.ts";
 import { installFromNpmTarball } from "../utils/install.ts";
-import { detectProviderError } from "../utils/providerErrors.ts";
+import { findProviderErrorMatch } from "../utils/providerErrors.ts";
 import { addSkill, installBundledSkills } from "../utils/skills.ts";
 import {
   DEFAULT_MAX_RETAINED_BYTES,
@@ -1032,10 +1032,10 @@ async function runOpenCode(params: RunParams): Promise<AgentResult> {
         recentStderr.push(trimmed);
         if (recentStderr.length > MAX_STDERR_LINES) recentStderr.shift();
 
-        const providerError = detectProviderError(trimmed);
-        if (providerError) {
-          lastProviderError = providerError;
-          log.info(`» provider error detected (${providerError}): ${trimmed.substring(0, 500)}`);
+        const match = findProviderErrorMatch(trimmed);
+        if (match) {
+          lastProviderError = match.label;
+          log.info(`» provider error detected (${match.label}): ${match.excerpt}`);
         } else {
           log.debug(trimmed);
         }