feat: skill injection — teach agents how to use spawn on recursive VMs (#2989)

When `--beta recursive` is active, a new "Spawn CLI" setup step injects
agent-native instruction files teaching each agent how to use the `spawn`
CLI to create child VMs. Skill files live in `skills/` at the repo root
and use each agent's native format (YAML frontmatter for Claude/Codex/
OpenClaw, plain markdown for others, append mode for Hermes).

- Add `skills/` directory with 8 agent-specific skill files
- Add `spawn-skill.ts` module with path mapping, file reading, and injection
- Register "spawn" as a conditional setup step gated by `--beta recursive`
- Wire `injectSpawnSkill()` into orchestrate.ts postInstall flow
- Add 52 tests covering path mapping, append mode, file existence, injection
- Bump CLI version to 0.26.0 (minor: new feature)

Co-authored-by: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

skills/opencode/AGENTS.md

@@ -0,0 +1,32 @@
+# Spawn — Create Child VMs
+
+You have the `spawn` CLI installed. Use it to provision cloud VMs with AI agents.
+
+## Usage
+
+```bash
+spawn <agent> <cloud> --headless --output json --prompt "task description"
+```
+
+## Agents: claude, codex, openclaw, zeroclaw, opencode, kilocode, hermes, junie
+## Clouds: hetzner, digitalocean, aws, gcp, sprite
+
+## Example
+
+```bash
+spawn claude hetzner --headless --output json --prompt "Implement the auth module"
+```
+
+Returns JSON: `{"status":"success","ip_address":"...","ssh_user":"root","server_id":"..."}`
+
+## Managing Children
+
+- `spawn list --json` — see running children
+- `spawn delete` — tear down a child VM
+- `spawn tree` — see the full spawn tree
+
+## Context
+
+- You are running inside a spawned VM (SPAWN_DEPTH is set)
+- Cloud credentials are pre-configured — no auth prompts
+- OpenRouter billing is shared with the parent