fix(agent): count only dispatchable files reviewed

[ousamabenyounes]

Description

Makes files_reviewed report the number of dispatchable files instead of the raw retained diff slice length. Deleted diffs remain available through Diffs() for context and line resolution, but they are no longer counted as reviewed files.

Type of Change

• Bug fix (non-breaking change that fixes an issue)
• New feature (non-breaking change that adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Refactoring (no functional changes)
• Documentation update
• CI / Build / Tooling

How Has This Been Tested?

• make test passes locally
• Manual testing (describe below)

Test verification (RED → GREEN)

RED on upstream-base behavior with the new regression test:

--- FAIL: TestAgentFilesReviewedCountsDispatchableDiffs (0.02s)
    coverage_test.go:78: FilesReviewed() = 3, want 2
FAIL
FAIL	github.com/open-code-review/open-code-review/internal/agent	0.027s

GREEN after the fix:

ok  	github.com/open-code-review/open-code-review/internal/agent	0.016s

Full local Go CI replay in golang:1.26.5:

gofmt -s -l .
go mod tidy && git diff --exit-code -- go.mod go.sum
go vet ./...
go test -v -race -count=1 -coverprofile=coverage.out ./...
Total coverage: 81.5%
go build -o /dev/null ./cmd/opencodereview

Cross-compile check started in golang:1.26.5 and built linux/arm64, darwin/amd64, darwin/arm64, and windows/amd64; the final target was blocked by runner disk exhaustion while compiling dependencies (no space left on device).

Checklist

• My code follows the project's coding style (go fmt, go vet)
• I have performed a self-review of my code
• I have added tests that prove my fix is effective or my feature works
• New and existing unit tests pass locally with my changes
• I have updated the documentation accordingly (if applicable)
• I have signed the CLA

Related Issues

Fixes #480

[CLAassistant]

All committers have signed the CLA.

[github-actions]

✅ OpenCodeReview: No comments generated. Looks good to me.

[lizhengfeng101]

LGTM