fix(ci): resolve all test failures + complete rename compat layer

Sisyphus-authored fixes across 15 files:

- plugin-identity: align CONFIG_BASENAME with actual config file name
- add-plugin-to-opencode-config: handle legacy→canonical name migration
- plugin-detection tests: update expectations for new identity constants
- doctor/system: fix legacy name warning test assertions
- install tests: align with new plugin name
- chat-params tests: fix mock isolation
- model-capabilities tests: fix snapshot expectations
- image-converter: fix platform-dependent test assertions (Linux CI)
- example configs: expanded with more detailed comments

Full suite: 4484 pass, 0 fail, typecheck clean.

src/shared/legacy-plugin-warning.ts

@@ -0,0 +1,57 @@
+import { existsSync, readFileSync } from "node:fs"
+
+import { parseJsoncSafe } from "./jsonc-parser"
+import { getOpenCodeConfigPaths } from "./opencode-config-dir"
+import { LEGACY_PLUGIN_NAME, PLUGIN_NAME } from "./plugin-identity"
+
+interface OpenCodeConfig {
+  plugin?: string[]
+}
+
+export interface LegacyPluginCheckResult {
+  hasLegacyEntry: boolean
+  hasCanonicalEntry: boolean
+  legacyEntries: string[]
+}
+
+function getOpenCodeConfigPath(): string | null {
+  const { configJsonc, configJson } = getOpenCodeConfigPaths({ binary: "opencode", version: null })
+
+  if (existsSync(configJsonc)) return configJsonc
+  if (existsSync(configJson)) return configJson
+  return null
+}
+
+function isLegacyPluginEntry(entry: string): boolean {
+  return entry === LEGACY_PLUGIN_NAME || entry.startsWith(`${LEGACY_PLUGIN_NAME}@`)
+}
+
+function isCanonicalPluginEntry(entry: string): boolean {
+  return entry === PLUGIN_NAME || entry.startsWith(`${PLUGIN_NAME}@`)
+}
+
+export function checkForLegacyPluginEntry(): LegacyPluginCheckResult {
+  const configPath = getOpenCodeConfigPath()
+  if (!configPath) {
+    return { hasLegacyEntry: false, hasCanonicalEntry: false, legacyEntries: [] }
+  }
+
+  try {
+    const content = readFileSync(configPath, "utf-8")
+    const parseResult = parseJsoncSafe<OpenCodeConfig>(content)
+    if (!parseResult.data) {
+      return { hasLegacyEntry: false, hasCanonicalEntry: false, legacyEntries: [] }
+    }
+
+    const legacyEntries = (parseResult.data.plugin ?? []).filter(isLegacyPluginEntry)
+    const hasCanonicalEntry = (parseResult.data.plugin ?? []).some(isCanonicalPluginEntry)
+
+    return {
+      hasLegacyEntry: legacyEntries.length > 0,
+      hasCanonicalEntry,
+      legacyEntries,
+    }
+  } catch {
+    return { hasLegacyEntry: false, hasCanonicalEntry: false, legacyEntries: [] }
+  }
+}