feat!: installation only from a release, into an empty folder; upstream merge/verify and private-instance.md removed, dist adopt, shell-neutral instructions (#153)

Files changed:
- .gitea/workflows/ci.yml
- .gitea/workflows/release.yml
- AGENTS.md
- CHANGES.md
- DEVELOPMENT.md
- EVALS.md
- INSTALL.md
- README.md
- VERSION
- docs/ownership-and-templates.md
- instructions/CONTRACT.md
- instructions/bootstrap.md
- instructions/dev/dev-setup.md
- instructions/dev/stack-dev/SKILL.md
- instructions/gates.md
- instructions/ingest-large-tree.md
- instructions/kb-profiles.md
- instructions/mcp-read-server.md
- instructions/migrations/3.0.0-authoring-conventions.md
- instructions/preflight.md
- instructions/private-instance.md
- instructions/session-setup.md
- instructions/setup-instance.md
- instructions/upgrade-instance.md
- tools/CONTRACT.md
- tools/README.md
- tools/chemenu/cli.py
- tools/chemenu/cli_contract.py
- tools/chemenu/commands/dist_cmd.py
- tools/chemenu/commands/docs_verify.py
- tools/chemenu/commands/doctor.py
- tools/chemenu/commands/git_publish.py
- tools/chemenu/commands/upstream_cmd.py
- tools/chemenu/commands/work_cmd.py
- tools/chemenu/config.py
- tools/chemenu/ownership.py
- tools/chemenu/tests/test_cli.py
- tools/chemenu/tests/test_dist_cmd.py
- tools/chemenu/tests/test_instructions_shell.py
- tools/chemenu/tests/test_preflight.py
- tools/chemenu/tests/test_preflight_pwsh.py
- tools/chemenu/tests/test_run_budget.py
- tools/chemenu/tests/test_upstream_cmd.py
- tools/chemenu/toc.py
- tools/preflight.ps1
- tools/preflight.sh

instructions/CONTRACT.md

@@ -179,6 +179,18 @@ Scaffold with `tools/wikitool new instruction --name "<name>"`; the contract is
   at all. If that is worth preserving, it is a concept page under `kb/concepts/`, linked from
   here. Where the line runs, and how to test a passage against it: below.
 - **State scope boundaries.** When does this *not* apply, and what to do instead.
+- **A command block reads the same in every shell.** Depending on the harness, an instruction
+  runs under bash, Git Bash or PowerShell 7. A command in a fenced block is a `tools/wikitool`
+  or `git` call, or the preflight's own call per platform - never syntax only one shell reads: no
+  heredoc, no `export`, no `$(...)` or `$VAR`, no inline `VAR=value command`, no `&&`, no `for`
+  loop, no `cp`, `cat >`, `sha256sum`, `curl` or `tar`. A step that needs one of them gets a
+  `wikitool` command instead, or leaves the file work to the agent's own file tools. Two places
+  are exempt, each with one line per shell: setting the session id
+  ([session-setup.md](session-setup.md)) and downloading the preflight before an instance exists
+  ([setup-instance.md](setup-instance.md) step 0). Migration documents under
+  `instructions/migrations/` belong to the release they shipped with and are not rewritten. The
+  stack's own instructions are held to this by a test in the origin repository; what an instance
+  writes for itself is its own decision.
 - **Write it in English, and let the agent speak the instance's language.** Both rules, and the
   line between prose and quoted vocabulary, are stated once in
   [AGENTS.md § File naming](../AGENTS.md#file-naming). They are named here because this is the