[claude-agent-service] Migrate all pipelines from DevVM SSH to K8s HTTP

## Context

The claude-agent-service K8s pod (deployed 2026-04-15) provides an HTTP API
for running Claude headless agents. Three workflows still SSH'd to the DevVM
(10.0.10.10) to invoke `claude -p`. This eliminates that dependency.

## This change

Pipeline migrations (SSH → HTTP POST to claude-agent-service):
- `.woodpecker/issue-automation.yml` — Vault auth fetches API token instead
  of SSH key; curl POST /execute + poll /jobs/{id} replaces SSH invocation
- `scripts/postmortem-pipeline.sh` — same pattern; uses jq for safe JSON
  construction of TODO payloads
- `.woodpecker/postmortem-todos.yml` — drop openssh-client from apk install
- `stacks/n8n/workflows/diun-upgrade.json` — SSH node replaced with HTTP
  Request node; API token via $env.CLAUDE_AGENT_API_TOKEN (added to Vault
  secret/n8n)

Documentation updates:
- `docs/architecture/incident-response.md` — Mermaid diagram: DevVM → K8s
- `docs/architecture/automated-upgrades.md` — pipeline diagram + n8n action
- `AGENTS.md` — pipeline description updated

## What is NOT in this change

- DevVM decommissioning (still hosts terminal/foolery services)
- Removal of SSH key secrets from Vault (kept for rollback)
- n8n workflow import (must be done manually in n8n UI)

[ci skip]

Co-Authored-By: Claude Opus 4 (1M context) <noreply@anthropic.com>

.woodpecker/postmortem-todos.yml

@@ -17,7 +17,7 @@ steps:
   - name: parse-and-implement
     image: python:3.12-alpine
     commands:
-      - apk add --no-cache jq curl git openssh-client
+      - apk add --no-cache jq curl git
       - sh scripts/postmortem-pipeline.sh
 
   - name: notify-slack