docs: point the install one-liner at v0.4.0 #368

docs: point the install one-liner at v0.4.0

Component: the edited files are existing files at the repository root, in docs/ and in scripts/. No new files.

v0.4.0 is published (https://github.com/DanielNetzer/forgecell/releases/tag/v0.4.0), built from main 721ef75. Its anonymous install is verified. This follow-up is the same as #284/#285 for v0.3.1.

Change (text only; replace every v0.3.1 reference that names the current pinned release):

• README.md:
  • The "Early release" note says the downloadable release is v0.4.0. Replace the sentence "GitHub is the only tracker in a release; GitLab, Jira and outside reviewers are available in the source on main and not yet in a release." with one saying v0.4.0 also ships the plugin runtime with bundled GitLab (forge), Jira (tracker) and forgecomments (reviewer) plugins, and link to lab/plugins/README.md.
  • Change the installer summary line, the one-liner URL (.../download/v0.4.0/install.sh), "pins the published public-core v0.4.0", and "The v0.4.0 release defaults to…".
• docs/install.md: change line 3, the one-liner URL, the Lab-directory sentence, the "Versions, upgrade and rollback" paragraph and the "pinned installer uses … v0.4.0 tag" line.
  • In the versions paragraph, replace "v0.3.1 is the first release that drives the whole loop from forgecell ui." with "v0.4.0 adds plugins: other trackers, forges and reviewers through pinned MCP plugins, and the plugin new/pin/test contributor kit."
  • Keep the existing historical v0.2.0-preview.2 provenance text.
• scripts/test-native-installer.py: public_url points at v0.4.0.
• Leave everything else byte-identical. The one-liner command text changes only in its version.

Scope: README.md, docs/install.md, scripts/test-native-installer.py.

Done when

• No remaining reference in these three files names v0.3.1 as the current release.
• sh scripts/check-repo-hygiene.sh and its test pass.

Plan checks for this slice

Use only the Formula baseline checks; no candidate Go test is needed because no Go code changes. Every plan check leaves reviewedAcceptance and independentProvenance empty.