feat(install): the installers take --base, --team and --personal and never ask; install.ps1 never exits the user's session; CI runs both on every platform - #984

Summary

Lanes 15, 16 and 17 of #949, PR B of four (ruled 2026-09-29: "We'll four PRs: A, B, #959, and then C.").

• The installers take --base, --team and --personal: a layer is registered when given, named as skipped when not — the script never asks #950, the layer flags: --base, --team and --personal (-Base, -Team, -Personal; DEVLORE_BASE, DEVLORE_TEAM, DEVLORE_PERSONAL) register each layer with the writ just installed, writ repo set <layer> <loc> --unattended, base first. The installers never ask; a layer not given is skipped and named last with its command. writ's output is left as it prints it; the summary follows it. Long options, the standard headers, Apache-2.0, GH_TOKEN sent when set, unknown arguments refused, and PATH advice that names the directory installed into.
• install.ps1's exit closes the user's PowerShell window: under irm | iex and the script-block form, exit ends the session, not the script #965, no exit: install.ps1 returns from -Help and throws on failure, so irm | iex and the script-block form leave the user's session open with the message on screen. Run as a file, a failure still exits 1. Native calls run under 'Continue', so a Windows PowerShell 5.1 user who redirects the streams keeps the install.
• The getting-started guide and the README install from assets the releases do not publish, and show neither the one command nor the three flags #946, the documents: the README and the getting-started guide lead with the one command for bash and PowerShell (the script-block form preferred), a table of the flags, and the steps by hand. The guide's repository and deploy sections stay as develop has them; the first-use scenario goes to Schedule: writ works on a machine that has never seen it #894's replan (ruled 2026-09-30).
• CI: .github/workflows/installers.yaml runs scripts/Test-InstallScript.sh on macos-latest, macos-15-intel, ubuntu-latest and ubuntu-24.04-arm (on macOS with macOS's own bash 3.2 and BSD tools), and scripts/Test-InstallScript.ps1 on windows-latest and windows-11-arm under Windows PowerShell 5.1 and PowerShell 7. Both scripts run by hand, in a scratch account.

Also: wiki/Releasing.md says the release copies both installers; the README's Noble Factor link points at https://github.com/NobleFactor; PR A's plan (#962) takes its last tick and is complete. Found and filed: #982, #983.

• Plan — docs/plans/feature/950-installers-take-base-team-and.md. It stays active: its last boxes are this PR's installers jobs and, after the merge, the site's copies of both installers.

Resolves #950
Resolves #965
Resolves #946

Test plan

• bash, in scratch accounts: flags through the pipe, twice (unchanged); a URL layer; DEVLORE_BASE alone and flag over variable; no flags with and without a terminal, identical; refusals; 36 of 37 checks, the 37th a dead noblefactor.com link, since repointed
• pwsh 7: the script-block form, irm | iex, -Help, a forced failure under both forms leaving the session open, -File exit codes; develop's install.ps1 ends the session on the same failure
• scripts/Test-InstallScript.sh, 14 of 14, and scripts/Test-InstallScript.ps1 under pwsh 7, 12 of 12, on Linux arm64; the host's devlore files and the checkout untouched
• An eight-agent review of the CI tests for macOS, Windows PowerShell 5.1, GitHub Actions and writ on Windows: two defects confirmed and fixed (5.1's redirected native stderr, 5.1's Remove-Item on directory links)
• Gates: star lint shell, the PowerShell gate, ASCII only, 0 exit nodes in install.ps1, guide frontmatter; every address the documents add answers 200
• This PR's installers jobs, all six runners, both PowerShell editions
• After the merge: the site's install.sh and install.ps1 equal develop's

Time

Computed by the algorithm of noblefactor-ops#188 at merge.