feat: add Ash declarative data layer for GitHub resources

[jordangarrison]

Summary

• Add Ash Framework 3.19 as a declarative data layer, replacing hand-rolled Models module with 10 typed Ash resources (WorkflowRun, Job, Step, Repository, Pull, Branch, Release, User, UserPR, UserCommit)
• Create Greenlight.GitHub Ash Domain with code interface functions (list_workflow_runs/2, list_jobs/3, list_pulls/2, etc.) providing a single entry point for all GitHub data access
• Migrate all consumers (Poller, WorkflowGraph, UserInsightsServer, PipelineLive, DashboardLive, RepoLive) from direct Client calls to the Ash domain interface
• Simplify Client to a pure HTTP transport layer returning raw API maps, with all data shaping handled by ManualRead action modules
• Remove Models module — all struct definitions now live in Ash resources with proper type declarations

Architecture

LiveViews / GenServers
        ↓
  Greenlight.GitHub (Ash Domain)
        ↓
  ManualRead Actions (data shaping)
        ↓
  GitHub.Client (HTTP transport, raw maps)
        ↓
  GitHub REST API

Changes

Area	Files	What
Dependency	mix.exs, mix.lock, config/config.exs	Add {:ash, "~> 3.19"}, configure domain
Resources	lib/greenlight/github/{step,job,workflow_run,repository,pull,branch,release,user,user_pr,user_commit}.ex	10 Ash resource definitions
Domain	lib/greenlight/github/domain.ex	Greenlight.GitHub Ash Domain with code interfaces
Actions	lib/greenlight/github/actions/*.ex	11 ManualRead actions + parsing helpers
Migration	poller.ex, workflow_graph.ex, user_insights_server.ex, *_live.ex	Use domain instead of Client
Cleanup	models.ex deleted, client.ex simplified	Remove Models, Client returns raw maps
Nix	nix/package.nix	Updated dep hash for ash
Tests	domain_test.exs added, models_test.exs removed, others updated	63 tests, 0 failures

Test plan

• mix compile --warnings-as-errors passes
• mix test — 63 tests, 0 failures
• nix build .#default succeeds
• nix build .#dockerImage succeeds
• Manual: Dashboard loads user insights and org repos
• Manual: Repo view shows pulls, branches, releases, commits
• Manual: Pipeline DAG viewer loads and updates in real-time