Add support for $GITHUB_ARTIFACTS environment files

[bdehamer]

Summary

Adds support for two new per-job environment files that let workflow steps declare build artifacts and read back the aggregated set:

• $GITHUB_ARTIFACTS (write side) — a step appends artifact references (local file paths or OCI subject references). The runner parses each entry, validates it, computes a digest for local files, and aggregates the results at job scope.
• $GITHUB_ARTIFACTS_LIST (read side) — the runner populates a JSON document enumerating every artifact subject declared so far in the job, so downstream steps can consume the canonical set.

The feature is gated behind the actions_runner_allow_artifacts_file feature flag (with an ACTIONS_RUNNER_ALLOW_ARTIFACTS_FILE environment-variable fallback) and is off by default.

Changes

New files

• CreateArtifactsFileCommand.cs — Write-side handler for $GITHUB_ARTIFACTS. Parses and validates each entry, computes SHA-256 digests for local files (resolved relative to the workspace root), and aggregates subjects at job scope with de-duplication, conflicting-digest detection, a 1 MiB file-size cap, and a 500-artifact aggregate cap.
• ArtifactsListFileCommand.cs — Read-side handler that populates $GITHUB_ARTIFACTS_LIST with a versioned JSON document ({ version, subjects: [{ name, digest, kind }] }).
• ArtifactSubject.cs — Subject model (Name, Digest, Kind) shared by both commands.
• CreateArtifactsFileCommandL0.cs, ArtifactsListFileCommandL0.cs — Unit tests covering parsing, validation, digest computation, aggregation, de-dup/conflict handling, and list serialization.

Modified files

• FileCommandManager.cs — Adds a PopulateInitialContents hook (default no-op) so the list command can seed $GITHUB_ARTIFACTS_LIST before step execution; covered by FileCommandManagerL0.cs.
• ExtensionManager.cs — Registers the two new file commands.
• GlobalContext.cs / ExecutionContext.cs — Adds the job-scoped ArtifactSubjects aggregate, keyed by canonical subject name.
• GitHubContext.cs — Exposes the artifacts and artifacts_list context entries.
• Constants.cs — Adds the AllowArtifactsFile feature flag and the artifact command error messages.

Key design decisions

• Workspace-relative path resolution — Relative file paths in $GITHUB_ARTIFACTS are always resolved against GITHUB_WORKSPACE rather than a step's working-directory, matching the precedent set by hashFiles() and problem matchers.
• Job-scoped aggregation — Subjects are accumulated across all steps in the job and de-duplicated by canonical name; re-declaring a name with a different digest is a hard error.
• Feature-flagged and off by default — Gated by actions_runner_allow_artifacts_file with an env-var fallback, so the behavior is opt-in until rollout.