Remove packages/symphony leftovers (#780)

## Summary
- remove stale root workspace/docs references to the reverted
`packages/symphony` subtree
- restore the pre-subtree external `symphony` flake input as the
provider for `pkgs.symphony-room-server`
- keep the existing `images/dev/symphony-codex` image and its eval tests
intact

## Validation
- `nix flake metadata --json`
- `python -m json.tool flake.lock`
- `git diff --check`
- `rg -n "packages/symphony|smypohny|Smypohny" Cargo.toml README.md
flake.nix flake.lock lib/default.nix lib/overlay.nix tests/default.nix
images/dev/symphony-codex/default.nix || true`

Note: `nix build .#ciChecks.x86_64-linux.eval --no-link --show-trace` no
longer reports the missing `packages/symphony` path; on this aarch64
host it now stops on ordinary x86_64 Rust derivation platform mismatch.


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> [!NOTE]
> ### Replace local `packages/symphony` with external symphony flake
input
> - Removes the in-tree `packages/symphony` workspace and the
`symphonyFor` helper that built it locally with a custom nixpkgs import.
> - Adds a `symphony` flake input pointing to
`github:indexable-inc/symphony/main`, threaded through `flake.nix`,
`lib/default.nix`, and `lib/overlay.nix`.
> - `symphony-room-server` in the overlay now resolves from
`symphony.packages."${packageSystem}".room-server` instead of the local
build.
> - Behavioral Change: downstream consumers must use the external flake
input rather than the previously in-tree symphony packages.
>
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b6976c3.</sup>
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Author: harivansh-afk

Cargo.toml

@@ -80,14 +80,8 @@ members = [
 # so it owns its own Cargo.toml + Cargo.lock (it carries its own `[workspace]`).
 # Excluding it here keeps the root lockfile from pulling it in, so a root-lock
 # bump never recompiles it; its Nix build keys only on its own folder.
-#
-# packages/symphony is the tucked-in subtree from indexable-inc/symphony. It
-# carries its own nested Cargo workspace (`packages/symphony/Cargo.toml`) plus
-# its own `Cargo.nix.lock` for room-server's Nix build, so the same exclude
-# pattern keeps it isolated from the root lockfile.
 exclude = [
   "packages/nix-cargo-unit",
-  "packages/symphony",
 ]
 resolver = "3"
 

README.md

@@ -66,7 +66,6 @@ nix build .#minecraft   # realize one image closure
 ## Layout
 
 - [`packages/`](packages/) repo-owned tools (search, PTY driver, agent loops, MCP server, the `reel` demo recorder).
-- [`packages/symphony/`](packages/symphony/) the multiplayer Codex agent runtime tucked in from indexable-inc/symphony: an Elixir orchestrator, a Rust `room-server` (HTTP/3 + WebTransport SFU), and a Tauri desktop client. Self-contained nested Cargo workspace; the room-server is exposed through `pkgs.symphony-room-server` for [`images/dev/symphony-codex`](images/dev/symphony-codex/).
 - [`images/`](images/) runnable NixOS systems packaged as OCI archives.
 - [`modules/`](modules/) opt-in NixOS service modules, auto-discovered.
 - [`lib/`](lib/) shared helper and builder API.