fix(dotprompt-go): paginate directory store listings

[rksharma-owg]

Go DirStore.List and ListPartials ignore the returned cursor and repeat the first page. Honor cursor offsets after the existing filtering and sorting, return an empty cursor on the final page, and reject invalid cursors.

Add filesystem regression tests for prompts and partials covering page progression, variants, nested names, terminal pages, invalid cursors, and limit boundaries. Document cursor usage and the absence of snapshot guarantees when files change between calls.

Fixes #500.

Validation

Tested commit: a83a2163369b37b1bcc293a5b433f761378e75d0; baseline: 964b3a83c49a357d4a1b215bd307d513f2a38aef.

• Regression tests fail against unchanged production baseline and pass with this patch.
• Local Go tests, race/coverage, vet, build, lint, formatting, module hygiene, license and TODO checks pass.
• Cloud Go 1.25.14/1.26.8 builds and tests pass. Bazel Java 17/21 builds and all 68 test targets pass for both patch and baseline.
• Cloud compatible-Go lint, license and TODO checks pass.

Patch/baseline preflight · Toolchain diagnostics

Existing CI failures

Preflight is not fully green. These failures also occur on unchanged baseline:

• Go 1.26 vulnerability scanning reports GO-2026-5970 in the existing golang.org/x/text 0.36.0 dependency.
• Go 1.25 cannot install latest govulncheck: x/vuln 1.8.0 requires Go 1.26.
• Stable Go resolves to 1.27.1, while pinned golangci-lint 2.10.1 was built with Go 1.26 and panics. The same linter passes for this patch with Go 1.26.8.

Dependency and CI configuration changes are excluded from this focused pagination fix.