feat: instructionsFilePath config field + skill bundle materialization

- config-schema: add instructionsFilePath UI field (Core group, text type)
- server/index.ts: set supportsInstructionsBundle=true, instructionsPathKey="instructionsFilePath"
- execute.ts: read instructionsFilePath file + desired skill markdown files from PVC; pass to buildJobManifest as instructionsContent / skillsBundleContent
- job-manifest.ts: accept instructionsContent + skillsBundleContent in JobBuildInput; prepend both to prompt via joinPromptSections; add instructionsChars + skillsBundleChars to promptMetrics
- index.ts: document instructionsFilePath and skills injection in agentConfigurationDoc
- CLAUDE.md: document skill materialization (ephemeral mode) and instructionsFilePath field
- Bump version to 0.1.18

Co-Authored-By: Paperclip <noreply@paperclip.ing>

src/server/index.ts

@@ -20,8 +20,8 @@ export function createServerAdapter(): ServerAdapterModule {
     supportsLocalAgentJwt: true,
     agentConfigurationDoc,
     getConfigSchema,
-    supportsInstructionsBundle: false,
-    instructionsPathKey: undefined,
+    supportsInstructionsBundle: true,
+    instructionsPathKey: "instructionsFilePath",
     requiresMaterializedRuntimeSkills: false,
     sessionManagement: getAdapterSessionManagement("opencode_local") ?? {
       supportsSessionResume: true,