docs(agents): add shared engineering guidance

[stozo04]

Why

Require agents to read the shared reuse-first engineering prompt before project work. The owner requested the same root filename across all listed projects.

Scope

• Add Engineering-Prompt.md and first-line links in AGENTS.md and CLAUDE.md. Preserve the existing pointer content.
• Allow only that exact additional root filename in the documentation placement workflow and update the policy references.
• Preserve the supplied prompt wording, with a final newline for Markdown lint.

Verification

• Passed the full pre-PR sweep with -RerunTests -SkipConnected -SkipInspectCode on the final clean commit. The branch includes YAML, so the documentation-only shortcut was not used.
• Debug and release builds passed. All 647 JVM tests executed with zero failures or errors. Android lint had zero hard findings; 24 native-library alignment entries passed.
• Markdown, tables, links, skill synchronization, script self-checks, spelling, JSON, dictionary synchronization, and secret/hygiene checks passed.
• Exercised the actual workflow case statement with nine placement cases, including three rejected paths. Independent diff review found no actionable issues.

The first sweep had two UncaughtExceptionsBeforeTest failures in OpenLoopViewModelTest, with background work reaching a reset Main dispatcher. The affected class passed separately, then the full forced test rerun passed without application or test changes. This does not claim to fix the underlying intermittent teardown behavior.

Device tests and installed-app loops, including onboarding, were skipped because the emulator was offline. IDE Inspect Code was skipped because no export was available. These checks remain unverified and should be completed before merge. No device screenshot is claimed.

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Note

Low Risk
Documentation and CI placement rules only; no runtime, auth, or app code paths change.

Overview
Introduces a shared reuse-first / ecosystem-first engineering prompt at root Engineering-Prompt.md so agents reconnaissance the codebase, dependencies, and ecosystem before writing custom code.

CLAUDE.md and AGENTS.md now open with a link to that file, then keep the existing pointers to docs/OPERATING_INSTRUCTIONS.md and docs/OPENLOOP_INSTRUCTIONS.md. The doc-layout CI workflow and docs policy (docs/README.md § Enforcement, DEFINITION_OF_DONE.md M1/M3, OPENLOOP_INSTRUCTIONS.md, STATIC_ANALYSIS.md) add Engineering-Prompt.md as the only new root Markdown exception and point error messages at the centralized allowlist.

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