[deep-report] defaultCacheIntegrityLevel string constant duplicates typed GitHubIntegrityNone enum

### Problem
`pkg/workflow/cache_integrity.go:16` defines `defaultCacheIntegrityLevel = "none"` as a bare string constant, duplicating the already-typed enum value `GitHubIntegrityNone GitHubIntegrityLevel = "none"` in `pkg/workflow/tools_types.go:299-300`.

### Why it matters
The default is not statically tied to the enum it's meant to represent — renaming or re-valuing `GitHubIntegrityNone` won't touch the untyped default, silently reintroducing a mismatch.

### Suggested fix
Change `defaultCacheIntegrityLevel`'s type to `GitHubIntegrityLevel` and assign it `GitHubIntegrityNone` directly.

### Data source
Typist code-quality analysis (Priority 3), live-verified against current `pkg/workflow/cache_integrity.go` and `pkg/workflow/tools_types.go` in this cycle — no matching open issue found.

**Suggested Agent:** general coding agent (Go)
**Estimated Effort:** Small (single constant retype + call-site check)

> [!WARNING]
>
> Firewall blocked 1 domain
>
> The following domain was blocked by the firewall during workflow execution:
>
> - `api.anthropic.com`
>
> To allow these domains, add them to the `network.allowed` list in your workflow frontmatter:
>
> ```yaml
> network:
> allowed:
> - defaults
> - "api.anthropic.com"
> ```
>
> See [Network Configuration](https://github.github.com/gh-aw/reference/network/) for more information.
>
>

> Generated by [🔬 Deep Report](https://github.com/github/gh-aw/actions/runs/34969464054) · claude · agent · 279.5 AIC · ⌖ 12.7 AIC · ⊞ 12.8K · [◷](https://github.com/search?q=repo%3Agithub%2Fgh-aw+is%3Aissue+%22gh-aw-workflow-call-id%3A+github%2Fgh-aw%2Fdeep-report%22&type=issues)
> - [x] expires on Sep 17, 2026, 4:50 AM UTC-08:00

Start with pkg/workflow/cache_integrity.go:16 and compare it with GitHubIntegrityNone in pkg/workflow/tools_types.go:299-300. Check the constant's call sites for type compatibility, then run the relevant Go package tests. Done means the default uses the typed enum value and the workflow package remains passing.

Written by the indexing model from the issue text.