fix: port prepareClaudePromptBundle flow to claude_k8s adapter (FAR-11)

K8s Job pods were starting without the Paperclip skill loaded, so agents
could not find their heartbeat procedure and reported "no issue content in
my workspace" on every wake. Root cause: claude_local materialises skills
into a PVC-backed prompt-bundle directory and passes --add-dir to Claude,
but claude_k8s did neither.

Changes:
- Add src/server/prompt-cache.ts with prepareClaudePromptBundle (ported
  from adapter-claude-local). Writes skill symlinks and the agent's
  instructions file into a content-addressed bundle directory under the
  shared PVC (/paperclip/instances/.../claude-prompt-cache/<hash>/).
- execute.ts: read desired skills and instructions file before building
  the Job manifest, then call prepareClaudePromptBundle and pass the
  resulting bundle to buildJobManifest.
- job-manifest.ts: accept optional promptBundle in JobBuildInput; when
  present, pass --add-dir <bundle.addDir> and use bundle.instructionsFilePath
  for --append-system-prompt-file. Also fix: skip --append-system-prompt-file
  on session resumes to avoid wasting tokens on re-injection.
- skills.ts: correct the detail string to reflect actual materialisation.
- job-manifest.test.ts: add 5 new tests covering --add-dir injection,
  bundle path preference, session-resume skipping, and fallback behaviour.

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

src/server/skills.ts

@@ -33,7 +33,7 @@ async function buildK8sSkillSnapshot(
     sourcePath: entry.source,
     targetPath: null,
     detail: desiredSet.has(entry.key)
-      ? "Injected via prompt bundle into ephemeral K8s Job pods."
+      ? "Materialized into the PVC-backed Claude prompt bundle before each K8s Job run."
       : null,
     required: Boolean(entry.required),
     requiredReason: entry.requiredReason ?? null,