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

## 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 #123

## Solution

```typescript
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)