Add PTM90 legacy LLM demo script

[Bol-C14]

Motivation

• Provide a runnable demo that preserves the legacy natural-language UX by mapping NL optimization requests into legacy target_values using an LLM.
• Give a concrete PTM90 example netlist and bundled model file so the demo can run include-safe in examples/.
• Help validate the legacy patch-only pipeline by wiring the mapped targets into LegacyPolicyLoopStrategy for a complete end-to-end example.

Description

• Add examples/run_legacy_ptm90_llm.py, a CLI demo that builds an LLMRequest to translate a free-text question into legacy target_values, calls LLMCallOperator, and runs LegacyPolicyLoopStrategy with the mapped targets.
• Add example circuit files examples/circuits/ptm90_basic_amp.sp and examples/circuits/ptm_90.txt to provide a PTM90 netlist and bundled model for safe .include usage.
• Provide a default mock LLM response (DEFAULT_MOCK_TARGETS) and request-building logic that sets response_schema_name="legacy_target_values" so the demo can run with --provider mock out of the box.
• Add developer notes in docs/devlog/2026-02-15_1200_legacy_ptm90_demo.md describing how to run the demo and its purpose.

Testing

• No automated tests were run for this change (examples and docs only).

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Codex Task