add docs for agents

[nihui]

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[Copilot]

Pull request overview

Adds a new “agent-facing” documentation set to help contributors/automation navigate ncnn’s architecture, build/test workflow, and common implementation tasks (new operators + arch optimizations).

Changes:

• Introduces a top-level AGENTS.md index pointing to agent docs under docs/agents/.
• Adds task playbooks for adding operators (ncnn + PNNX) and implementing optimizations (x86 SIMD, Vulkan, cross-arch).
• Documents key internals (data structures, dispatch logic, PNNX pipeline) and practical build/test guidance.

Reviewed changes

Copilot reviewed 11 out of 11 changed files in this pull request and generated 5 comments.

Show a summary per file

File	Description
docs/agents/task-x86-optimization.md	Step-by-step guide for adding x86 SIMD operator implementations.
docs/agents/task-vulkan-optimization.md	Guide for implementing Vulkan-accelerated layers and shaders.
docs/agents/task-cross-arch-optimization.md	Guidance for ARM/RISC-V/etc. optimizations and QEMU-based testing.
docs/agents/task-add-pnnx-operator.md	Instructions for adding new PNNX conversion/lowering support and tests.
docs/agents/task-add-operator.md	Checklist for adding a new ncnn layer, registration, and tests.
docs/agents/pnnx.md	High-level overview of PNNX architecture and conversion pipeline.
docs/agents/dispatch.md	Explains CPU/Vulkan dispatch and packing conversion behavior.
docs/agents/data-structures.md	Summarizes core structs/classes (Mat/Layer/Net/etc.) relevant to development.
docs/agents/code-style.md	Documents formatting/portability constraints and CI formatting workflow.
docs/agents/build-and-test.md	Build/test recipes, key CMake options, and coverage workflow notes.
AGENTS.md	Entry-point index for the new agent documentation set.

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