feat(init): support fully non-interactive setup via flags

[I-bangash]

Closes #510.

sandcastle init previously had four prompts with no flag overrides, which made it impossible to run in a non-TTY environment (CI, scripts, container builds). This PR adds an override flag for every remaining prompt so init runs end-to-end with stdin closed.

New flags

Flag	Values	Default
--sandbox-provider	docker, podman	interactive prompt
--backlog-manager	github-issues, beads	interactive prompt
--create-label	true, false	interactive prompt (default: yes)
--build-image	true, false	interactive prompt (default: yes)

All four flags follow the same Options.text + optional pattern as the existing --template, --agent, and --model flags: omit to be prompted, pass a value to skip the prompt.

When all flags are provided, init skips every clack prompt and runs to completion. Existing interactive behavior is unchanged when flags are omitted.

Validation

• --sandbox-provider and --backlog-manager reject unknown values with an error listing valid choices.
• --create-label and --build-image reject values other than true/false with an error listing valid choices.

Non-interactive example

sandcastle init \
  --template blank \
  --agent claude-code \
  --sandbox-provider docker \
  --backlog-manager github-issues \
  --create-label true \
  --build-image true

Exits 0, scaffolds .sandcastle/, no prompts.

Tests

8 new cases in src/cli.test.ts:

• All new flags appear in init --help; removed flag names do not
• Invalid --sandbox-provider / --backlog-manager values error with the available list
• --create-label / --build-image with a non-boolean value error with true, false
• Fully non-interactive happy path with stdin closed → success
• --sandbox-provider podman writes a Containerfile (no Dockerfile)
• --backlog-manager beads skips the GitHub label step regardless of label flag

Verification

• npm run typecheck passes
• npm run build passes
• npx vitest run src/cli.test.ts — 22/22 pass
• Manual non-interactive run with stdin closed exits 0 and scaffolds correctly
• Changeset added (patch, per CLAUDE.md's pre-1.0 policy)
• README's sandcastle init options table updated

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