refactor(compute): gate ECS construct on compute_type context instead of comment toggle

[scottschreckengaust]

Summary

Replace the commented-out ECS construct in cdk/src/stacks/agent.ts with a proper compute_type context gate.

Problem

Currently, enabling ECS requires manually uncommenting code in agent.ts (lines ~46, ~524-533). This:

• Requires source code changes to change deployment topology
• Is invisible to the bootstrap/preflight system (commented code doesn't synthesize)
• Defeats the compute-variant architecture established in feat(bootstrap): custom bootstrap template generation #123

Solution

const computeType = app.node.tryGetContext('compute_type') ?? 'agentcore';

// In the stack:
if (computeType === 'ecs') {
  new EcsAgentCluster(this, 'EcsAgentCluster', { vpc, ... });
}

The construct is always in source, always compilable, always tested. The context variable governs synthesis:

• mise //cdk:synth → agentcore only (default)
• mise //cdk:synth -- -c compute_type=ecs → agentcore + ECS resources

Acceptance criteria

• ECS construct uncommented and gated behind if (computeType === 'ecs')
• mise //cdk:synth (default) produces same template as today (no ECS resources)
• mise //cdk:synth -- -c compute_type=ecs produces template with ECS resources
• mise //cdk:test passes for both configurations
• No "uncomment to enable" prose remains in the file

Relationship to #120 stack

Prerequisite for #124 (resource-action-map) — the synth-coverage test needs to run with compute_type=ecs to verify ECS resource types are mapped. Without this issue, ECS resources never appear in the synthesized template.

Blocked by: #123 (compute-ecs policy must exist)
Blocks: #124 (ECS coverage in resource-action-map)