feat(templates): generalize boot flow and add template picker UI

[Dhi13man]

Why

ClawLess hardcodes GitClaw as the only agent — buildContainerPackageJson pins gitclaw: '1.1.4', buildWorkspaceFiles always mounts GitClaw config files, and sdk.ts branches on isGitclaw. This blocks #4 (OpenClaw support) because there's no way to register and boot a different agent.

This PR makes the boot flow template-driven so any agent can be registered and selected at startup.

Before / After

Before (main)	After (with 2 templates registered)

Before: Direct boot to loading screen — no choice of agent.
After: Picker overlay appears when multiple templates are registered. User clicks a card to select their agent.

Design Context

flowchart TD
    A[main.ts] -->|registry.size > 1?| B{Template Picker}
    A -->|registry.size <= 1| C[Boot default template]
    B -->|user clicks card| C
    C --> D[resolveTemplate]
    D --> E[mergeTemplateWithOptions]
    E --> F[boot: mount workspace + package.json from template]
    F --> G{template.configRequired?}
    G -->|yes| H[Show config panel, wait for API key]
    G -->|no| I[Launch agent directly]
    H --> I
    I --> J[startAgent with AgentConfig]

Loading

Trade-offs

• buildWorkspaceFiles now starts with empty tree instead of hardcoded defaults. Templates that don't declare workspace files get an empty workspace. Accepted because defaults now live in GITCLAW_TEMPLATE.workspace and flow through mergeTemplateWithOptions — GitClaw behavior is identical, new templates opt-in to exactly the files they need.
• startGitclaw() kept but deprecated rather than deleted. It has GitClaw-specific restart messaging that startAgent() doesn't replicate. Removing is safe but deferred to reduce risk.
• Picker uses textContent instead of innerHTML for template name/description. Prevents XSS if a malicious template is registered.

How to review

Order	File	What changed
1	src/workspace.ts	buildContainerPackageJson takes AgentConfig instead of hardcoding deps. buildWorkspaceFiles starts empty.
2	src/templates.ts	ContainerTemplate gains configRequired. GITCLAW_TEMPLATE declares its workspace files.
3	src/container.ts	boot() accepts agentConfig, passes it to package.json builder.
4	src/sdk.ts	isGitclaw branch replaced with template.configRequired. All agents use startAgent().
5	src/main.ts	Picker overlay — shows when >1 template, skips when 1.
6	index.html + src/style.css	Picker markup and styles.
7	src/*.test.ts	43 tests for all pure functions.

Test plan

• npx vitest run — 43 tests covering:
  • buildContainerPackageJson: agent config derivation, version defaulting, false/undefined, extraDeps merge
  • buildWorkspaceFiles: empty input, flat files, nested paths, multi-file merge
  • mergeTemplateWithOptions: per-key merge, agent override, tab dedup, startupScript fallback
  • parseTemplateYaml: scalars, agent blocks, env blocks, block scalars, validation
• Manual: npm run dev boots GitClaw identically (picker skipped with 1 template)
• Manual: register second template → picker renders both cards, selection triggers boot

Closes #4 (partial — template infrastructure)

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