docs(install): document upcoming Go-module migration + replace install command (#37)

This PR is README-only. The Go module-path migration
(github.com/Molecule-AI/molecule-cli → git.moleculesai.app/molecule-ai/
molecule-cli in go.mod + every internal import) is filed separately as
a cross-repo Go-module migration PR (parked follow-up on internal#37).

Changes here:
- Install section: keep the go install command but switch to the
  Gitea path + add a "Migration in progress" callout pointing at the
  build-from-source workaround until the Go-module PR lands.
- Releases link: removed the broken github.com/.../releases URL
  (releases pipeline being restored on Gitea Actions).
- RFC #10 link: rewritten to git.moleculesai.app form with note that
  the original issue lived on the suspended GitHub org.

Files NOT touched in this PR (left for the cross-repo Go-module PR):
- go.mod (module declaration)
- 25+ .go files with github.com/Molecule-AI/molecule-cli/internal/...
  import paths
- known-issues.md L96 (describes current module path; consistent with
  go.mod)
- CLAUDE.md L37 (declares current module path; consistent with go.mod)

These edits maintain consistency with go.mod while documenting the
upcoming change to users. Bundling them with the doc edit would create
a half-state where docs say one path and go.mod says another.

Refs: molecule-ai/internal#37, molecule-ai/internal#38

README.md

@@ -7,13 +7,31 @@ command, or a mock for CI).
 
 ## Install
 
+> **Migration in progress** (2026-05-07): the `github.com/Molecule-AI`
+> org was suspended on 2026-05-06 and is permanently gone. The
+> canonical SCM is now Gitea at
+> [`git.moleculesai.app/molecule-ai`](https://git.moleculesai.app/molecule-ai).
+>
+> The `go install` path below requires the Go module-path migration to
+> land first (separate cross-repo PR — see `internal#37` parked
+> follow-up). Until then, build from source:
+>
+> ```bash
+> git clone https://git.moleculesai.app/molecule-ai/molecule-cli.git
+> cd molecule-cli
+> go build -o molecule ./cmd/molecule
+> ```
+>
+> Once the Go module-path migration lands:
+
 ```bash
-go install github.com/Molecule-AI/molecule-cli/cmd/molecule@latest
+go install git.moleculesai.app/molecule-ai/molecule-cli/cmd/molecule@latest
 ```
 
-Or download a binary from [Releases](https://github.com/Molecule-AI/molecule-cli/releases).
 Releases ship Linux/macOS/Windows × amd64/arm64 archives plus a sha256
-checksums file (see `.goreleaser.yaml`).
+checksums file (see `.goreleaser.yaml`). Releases pipeline (Gitea
+Actions) is being restored as part of the post-suspension recovery; in
+the interim, build from source per the migration note above.
 
 ## Quick start — connect an external workspace
 
@@ -78,7 +96,7 @@ molecule config     view / set CLI defaults
 molecule completion generate shell completions
 ```
 
-The full M1 design is in [RFC #10](https://github.com/Molecule-AI/molecule-cli/issues/10).
+The full M1 design is in [RFC #10](https://git.moleculesai.app/molecule-ai/molecule-cli/issues/10) (originally filed on the suspended GitHub org; the issue may be re-filed on Gitea — check the issue tracker for the live discussion).
 
 ## License