Defer standalone-component migration and disable prefer-standalone lint rule

[fpigeonjr]

Description

Decides the standalone-component lint policy for legacy NgModules and closes out the human-in-the-loop architecture decision tracked in #584 (child of the ESLint lint-debt epic, #580).

Decision: defer the standalone-component migration rather than migrate or leave the rule as an unactionable warning. Rationale (evaluated and documented in AGENTS.md):

• Of the 190 root components flagged by @angular-eslint/prefer-standalone (3 in test-app), all but one are already standalone: false and registered via declarations: in one of 46 legacy NgModules. The single standalone: true outlier (SamAutocompleteComponent) is still consumed through declarations: in two modules, so it doesn't reflect real usage either.
• ESLint's own --fix for this rule mechanically strips standalone: false, flipping Angular's default to standalone: true. This was tried in Apply safe ESLint autofixes and lower the baseline #582/PR Apply safe ESLint autofixes and lower the warning baseline #675 and broke TestBed.configureTestingModule for every spec declaring one of these components — 827 failing tests. It's a behavior change, not a style fix.
• Several of the affected files (e.g. hierarchical.module, progress.module, autocomplete.module, progress.component, autocomplete-multiselect.component, radiobutton.component, text.component, toolbar.component) are in the frozen scripts/consumer-deep-imports.json contract — an uncoordinated migration risks breaking real downstream consumers, not just an internal refactor.

Changes:

• AGENTS.md — new "Standalone-component lint policy (deferred)" section under Lint, recording the decision and full rationale.
• eslint.config.mjs (root) — explicitly sets @angular-eslint/prefer-standalone to "off" (not just a warning), with an inline comment cross-referencing the AGENTS.md rationale and this issue.
• test-app/eslint.config.mjs — same treatment, referencing the root config's rationale.
• eslint-baseline.json — lowered to reflect the removed findings: root 1619 → 1369, test-app 4 → 1.

If a real standalone migration is ever undertaken, it will need its own epic, planned in consumer-safe slices (NgModule → imports: rewrites, spec updates, coordinated consumer rollout) — explicitly out of scope here.

Motivation and Context

Closes #584

Type of Change (Select One and Apply Label)

• Bug fix (non-breaking change which fixes an issue) → Apply bugfix label
• New feature (non-breaking change which adds functionality) → Apply enhancement label
• Breaking change (fix or feature that would cause existing functionality to change) → Apply breaking label
• Documentation / configuration update → Apply maintenance label

How to Test

1. npm ci && npm ci --prefix test-app
2. npm run lint:baseline → 0 errors, 1369 warnings (root), gate passes
3. npm --prefix test-app run lint:baseline → 0 errors, 1 warning (test-app), gate passes
4. npm --prefix test-app test → 152 test files / 1386 tests pass
5. npm run coverage:check → passes (coverage unaffected)
6. npm run format:check → passes
7. npm run validate:publish → passes (784 packed files, 67 consumer deep imports verified)
8. cd test-app && npm run build → succeeds

Expected result: All lint/test/coverage/publish/build gates pass. eslint-baseline.json reflects the lowered warning counts, and grep -c "prefer-standalone" eslint-report.json (either workspace) reports 0.

Screenshots (if appropriate)

N/A — no UI changes, lint policy and documentation only.

Checklist

• Branch name follows convention (e.g. gh-<number>-<slug>)
• PR title starts with a verb in the imperative mood
• I have self-reviewed my own code
• format:check passes (npm run format:check)
• lint passes (npm run lint)
• build passes (cd test-app && npm run build)
• Tests pass and coverage is reported (cd test-app && npm test)
• If this change requires a documentation update, I have updated it accordingly
• If there are dependent changes, they have been merged and published in downstream modules

[Copilot]

🟢 Approval recommended

The changes are limited to configuration/documentation updates and align with the stated decision, with only a minor wording/path clarity nit.

Pull request overview

Disables @angular-eslint/prefer-standalone in both workspaces and documents the rationale for deferring a standalone-component migration in this raw-source Angular library, while updating the lint warning baseline to reflect the removed findings.

Changes:

• Document the deferred standalone-component lint policy and rationale in AGENTS.md.
• Turn off @angular-eslint/prefer-standalone in root and test-app ESLint configs (instead of leaving it as warnings).
• Lower eslint-baseline.json warning counts to match the new lint output.

File summaries

File	Description
AGENTS.md	Adds a documented decision/rationale for deferring standalone migration and disabling prefer-standalone.
eslint.config.mjs	Disables @angular-eslint/prefer-standalone in the root workspace and records rationale inline.
test-app/eslint.config.mjs	Disables @angular-eslint/prefer-standalone in test-app, referencing the documented rationale.
eslint-baseline.json	Updates warning baselines to reflect the rule being disabled.

Review details

• Files reviewed: 4/4 changed files
• Comments generated: 1
• Review effort level: Lite

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