…jors

Two follow-ups from the first publish run:

## docs/INSTALL.md — new

A self-contained guide for end users who want to consume the framework
packages. Covers all three install paths the workflow produces:

* **NuGet.org** — the canonical path for stable releases. No setup —
  works out of the box on a fresh `dotnet --version`.
* **GitHub Packages** — for pre-release builds (anything with a
  `-preview` / `-rc` suffix) and internal mirror copies of stable
  versions. Walks through:
    * PAT creation with the minimal `read:packages` scope.
    * `dotnet nuget add source` (machine-global) vs. project-local
      `nuget.config` (commit-friendly with `%ENV%` substitution).
    * The CI-friendly path using the auto-provided `GITHUB_TOKEN`
      inside org-internal workflows; falling back to a stored PAT
      for cross-org consumers.
    * `--version` vs. `--prerelease` semantics — pre-release versions
      are not picked up by the default install.
* **Local project references** — for contributors developing
  against an unreleased change. `git clone` + `dotnet add reference`
  bypasses NuGet entirely.

Plus a *Listing every available version* section with the
`dotnet package search` invocations and a four-entry
*Troubleshooting* section covering the most common errors
(401, missing service index, package-not-found, package-downgrade).

## README + docs/README.md updates

* The "Build your first Firefly service" install step in the
  top-level README now distinguishes the two registries with a
  decision table and points at INSTALL.md for the GitHub Packages
  setup.
* INSTALL.md is added to the Documentation table in the README and
  to docs/README.md as the new first-step in the recommended reading
  order ("get the packages resolving in your project. Two minutes.").

## GitHub Actions — Node 24-ready majors

The first publish run produced a deprecation annotation:
> "Node.js 20 actions are deprecated … Actions will be forced to run
>  with Node.js 24 by default starting June 2nd, 2026."

Bumped every action in both workflows to its current major (each of
which ships on Node 22 / 24, resolving the deprecation):

| Action | Before | After |
|---|---|---|
| `actions/checkout` | `@v4` | `@v6` |
| `actions/setup-dotnet` | `@v4` | `@v5` |
| `actions/upload-artifact` | `@v4` | `@v7` |
| `softprops/action-gh-release` | `@v2` | `@v3` |

Major-tag floats (no patch / minor pin) — the GitHub Actions guidance
is to track the major so security patches land automatically. The
publish workflow is otherwise unchanged.

Build: 0 errors / 0 warnings. Tests: 346 / 346 still passing.