Trim AGENTS.md to invariants and task-specific routing

[morluto]

Fixes #2073.

Problem

AGENTS.md repeated SDK mechanics, deployment details, specialist commands, environment troubleshooting, and evaluation routing that already have authoritative owners elsewhere. That made routine agents load instructions unrelated to their task and created multiple copies of change-prone workflow guidance.

Solution

Keep the existing product narrative, semantic atomicity, operation-gap test, statelessness, bounded-result rules, backend-selection constraints, and mathematical boundedness obligation in the root guide.

Replace duplicated detail with decision-point routing:

• contribution guidance is loaded for validation, documentation, publication, release, or evaluation work;
• API, operation-library, transport, and deployment references are loaded only when that boundary changes;
• repository workflow retains the ordinary make check plus owner-lane handoff and the shared-checkout safeguards that prevent conflicting mutation.

The backend guidance now recommends an appropriate maintained mathematical library without biasing agents toward a fixed shortlist. Solvers and external tools remain permitted as private computational engines when the operation requires them.

Testing

• make docs-linkcheck — documentation commands and Markdown links passed.

• make check — Ruff, formatting, mypy, and 1,971 tests passed.

• git diff --check — passed.

• Specialist validation run: none; this changes agent documentation only.

Public contract impact

None. Operation IDs, native APIs, MCP contracts, and mathematical semantics are unchanged.

Public operation admission

Not applicable; no public operation changes.

Closure matrix

None.

Checklist

• make check passes
• Explicitly relevant specialist validation is listed above
• Harbor task or verifier changes ran the required preparation and validation (not applicable)
• Public operation changes include an owner-local admission decision (not applicable)
• Result semantics remain unchanged
• New shared abstractions replace duplication in surviving production paths (not applicable)