feat: prebuilt GHCR devcontainer image + simplified F5 launch

[piers-sinclair]

Summary

Ports the devcontainer improvements from DwellAround PRs #57 and #63 to the Dostar template.

Prebuilt GHCR image:

• New Dockerfile baking in all stable tooling: .NET 10, Node 22, pnpm, PostgreSQL client, Docker CLI, GitHub CLI, Azure CLI, dotnet-ef, Dostar.Cli, claude-code, Trivy, OpenGrep, Playwright Chromium, pre-warmed pnpm store and NuGet cache
• New devcontainer-image.yml CI workflow publishing ghcr.io/piers-sinclair/dostar-devcontainer on Dockerfile changes
• devcontainer.json: remove features and onCreateCommand (all tooling now in image)
• Delete devcontainer-lock.json (features lock, no longer applicable)
• configure-git-mounts.sh/.ps1: reference prebuilt image, add docker socket + playwright/pnpm/nuget named volumes, fix container_workspace to lowercase
• postCreate.sh: guard all tool installs (no-ops when image current), remove Trivy/OpenGrep wget blocks, add UI test pnpm install + Playwright browser check
• postStart.sh: add postStart.log, docker socket fix, 15-retry PG readiness wait, run migrations on every start
• postAttach.sh: fix trivy/opengrep repair hints, add playwright health check, add start log pointer

Simplified F5 launch:

• launch: prepare now only builds Dostar.Api (postStart handles migrations)
• New run: reset-db task for manually re-applying migrations after schema changes
• run: backend now depends on launch: prepare (not run: migrate directly)

First-time setup after merge

After CI publishes the first image, set the GHCR package to Public:
GitHub profile -> Packages -> dostar-devcontainer -> Package settings -> Change visibility -> Public

Test plan

• docker build succeeds locally
• Open devcontainer - no feature install step
• postCreate log shows tool guards firing (already on PATH)
• postStart log shows PG ready + migrations applied
• F5 starts backend and frontend without running migrations
• Terminal -> Run Task -> run: reset-db runs migrations manually
• health shows all tools green + both log pointers

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