[Init] backlog managers + 4 advanced templates + Podman variant

[pyadav]

Parent

#1

What to build

Extend sanddune init (slice #21) with:

1. Backlog managers — pluggable sources of tasks. v1 ships GitHub Issues and Beads. Each declares a Dockerfile snippet (installed via template argument substitution) and command placeholders for prompt templates. Backlog managers are an init-time concern only — no runtime API.
2. Advanced templates — simple-loop, sequential-reviewer, parallel-planner, parallel-planner-with-review. These exercise the live library API (run(), createSandbox(), createWorktree()) and reference the chosen backlog manager's commands.
3. Podman variant — when the user picks Podman during init, write a Containerfile instead of Dockerfile and use sanddune podman build-image (slice [Init] backlog managers + 4 advanced templates + Podman variant #22 — BuildImage).

Acceptance criteria

• Init prompts add: backlog manager (github | beads), template (blank | simple-loop | sequential-reviewer | parallel-planner | parallel-planner-with-review)
• Each backlog manager declares a Dockerfile snippet that gets installed via template argument substitution (e.g. {{BACKLOG_MANAGER_TOOLS}} in the Dockerfile is replaced with gh install steps for github, beads install steps for beads)
• Each backlog manager declares prompt-template command placeholders (e.g. LIST_TASKS_CMD, VIEW_TASK_CMD) that templates can reference
• Backlog managers are init-time only — no runtime API surface; they only affect scaffolded files
• All four advanced templates scaffold:
  • simple-loop — picks GitHub issues one by one and closes them
  • sequential-reviewer — implements issues one by one, with code-review step after each
  • parallel-planner — plans parallelizable issues, executes on separate branches, then merges
  • parallel-planner-with-review — same, with per-branch review before merge
• Podman variant: writes Containerfile instead of Dockerfile; scaffolded main.{mts,ts} uses podman() from @missingstudio/sanddune/sandboxes/podman; build step uses sanddune podman build-image
• Unit tests: each (backlog manager × template) combination scaffolds the expected file tree; Podman variant produces Containerfile
• Integration test: init with simple-loop + github against a temp repo with a fixture issue, run scaffolded main.{mts,ts} against a fake agent, verify the loop progresses
• bun test and bun run typecheck pass

Blocked by

• [Podman] bind-mount provider with SELinux + mounts + network #19
• [Init] sanddune init CLI — Docker + blank template #21