Add repo-level `defaultDriver` to `.swamp.yaml`

Problem

Repos that want every workflow to default to a specific execution driver have no way to declare that at the repo level. The driver-resolution chain in src/domain/workflows/execution_service.ts:1813–1818 is:

cli > step > job > workflow > "raw"

It never consults RepoMarkerData (src/infrastructure/persistence/repo_marker_repository.ts:42–62), which has no field for a default driver. The only ways to redirect every step away from "raw" today are:

• Set driver: on every workflow file (or every step)
• Pass --driver on every swamp workflow run invocation
• No env var exists for this — only SWAMP_DRIVERS_DIR for the directory path

A RepoMarkerData.datastore field already exists and follows the obvious pattern; defaultDriver would be the symmetric equivalent for the driver tier.

Proposed solution

1. Add defaultDriver?: string (and defaultDriverConfig?: Record<string, unknown>) to RepoMarkerData in src/infrastructure/persistence/repo_marker_repository.ts
2. In execution_service.ts:1813–1818, fall back to the repo marker before "raw". New chain: cli > step > job > workflow > repo > "raw"
3. Optionally: a CLI command to set it cleanly, mirroring swamp datastore setup

Alternatives considered

• CLI wrapper that injects --driver — only works when users invoke through the wrapper; raw swamp workflow run reverts to "raw".
• Setting driver: on every workflow — fragile and hostile to ergonomics for repos with many workflows.
• Per-definition default — the resolveDriverConfig function signature has an unused definition slot; populating it would help but does not cover the "all workflows in this repo" case.