.agent/.github: add topology2 agent design guidance

Add a canonical topology2 instruction file under .github/instructions and a
matching companion rule under .agent/rules. Document topology v2 structure,
reuse expectations, PCM and pipeline ID conventions, routing rules, platform
overrides, and target registration guidance for agents working in
tools/topology/topology2.

Our existing agent rules are in .agent, but github copilot doesn't understand
these yet, so add the companion rule.

Signed-off-by: Kai Vehmanen <kai.vehmanen@linux.intel.com>

Author: kv2019i

.agent/rules/topology2_design.md

@@ -0,0 +1,12 @@
+---
+trigger: glob
+glob: tools/topology/topology2/**
+---
+
+# Topology2 Design Rule
+
+When working in tools/topology/topology2, follow the canonical guidance in
+.github/instructions/topology2-design.instructions.md.
+
+Apply those instructions for topology structure, ID assignment, routing,
+platform overrides, and topology target registration.

.github/instructions/topology2-design.instructions.md

@@ -0,0 +1,98 @@
+---
+description: "Agent instructions for designing and updating ALSA topology v2 files in SOF"
+applyTo: 'tools/topology/topology2/**'
+---
+
+# Topology2 Design Instructions
+
+Use this guidance when creating or modifying files under tools/topology/topology2.
+These rules align with the topology2 README and capture expected design patterns for
+class-based ALSA topology v2 authoring.
+
+## Scope
+
+* Applies to topology2 .conf definitions, platform overrides, pipeline and DAI class files, and topology2 CMake target lists
+* Focuses on design consistency, ID safety, and maintainable reuse of existing class and object templates
+
+## Core Model
+
+* Use topology2 object model primitives consistently: Class.*, Object.*, Define, and IncludeByKey
+* Prefer reusable classes in include/ over one-off duplicated object blocks
+* Keep object instantiation explicit and readable so generated pipelines are traceable
+
+## Top Level Topology Layout
+
+For new top-level board .conf files, keep this order:
+
+1. Search directories
+2. Required class includes
+3. Define block with defaults
+4. IncludeByKey.PLATFORM overrides
+5. Feature-gated IncludeByKey blocks
+6. DAI, pipeline, and PCM objects
+7. Route definitions
+
+## Reuse Before New Base Files
+
+* Prefer extending an existing base input .conf with variable overrides from CMake targets
+* Add a new base .conf only when existing topologies cannot represent the use case cleanly
+* When adding a new target, use this tuple structure. In CMake quoted strings, escape each semicolon as `\;`.
+
+```text
+Logical tuple format:
+"input-conf;output-name;variable1=value1,variable2=value2"
+
+CMake string form:
+"input-conf\;output-name\;variable1=value1,variable2=value2"
+```
+
+## ID Conventions and Safety
+
+* Keep PCM IDs unique within a single topology
+* Keep pipeline indexes unique within a single topology
+* Pair FE and BE pipelines as N and N+1 where applicable
+* For SoundWire pipelines, follow index equals PCM ID times 10 unless a documented topology-specific exception exists
+* For HDMI pipelines, keep stride-10 layout with host at N0 and DAI at N1
+* When combining features such as SDW, PCH DMIC, HDMI, deep buffer, or compress, verify there are no ID collisions after overrides
+
+## Routing Rules
+
+* Connect FE mixin outputs to BE mixout inputs using Object.Base.route
+* Keep route naming and widget references aligned with topology naming patterns
+* Validate that each route endpoint maps to a declared widget in the same compiled topology
+
+## Widget Naming
+
+* Follow naming pattern type.pipeline-index.instance
+* Keep naming stable and descriptive for easier graph inspection and debug
+
+## Platform Overrides
+
+* Use IncludeByKey.PLATFORM for platform-specific Define overrides
+* Restrict platform-specific tuning to platform/intel/*.conf instead of duplicating board-level logic
+* Ensure platform keys remain consistent with the authoritative in-tree overrides under tools/topology/topology2/platform/intel/*.conf; current examples include tgl, adl, mtl, lnl, ptl, and nvl
+
+## CMake Target Placement
+
+Register targets in the correct file for platform generation:
+
+* Tiger Lake and Alder Lake: production/tplg-targets-cavs25.cmake
+* Meteor Lake: production/tplg-targets-ace1.cmake
+* Lunar Lake: production/tplg-targets-ace2.cmake
+* Panther Lake: production/tplg-targets-ace3.cmake
+* Nova Lake / sof-nvl-*: production/tplg-targets-ace4.cmake
+* i.MX8 platforms: production/tplg-targets-imx8.cmake
+* SDCA generic topologies: production/tplg-targets-sdca-generic.cmake
+* HDA generic: production/tplg-targets-hda-generic.cmake
+* Development and test topologies: development/tplg-targets.cmake
+
+If a target family is not listed above, use the existing tplg-targets-*.cmake
+file that already contains similar topologies as the source of truth, and keep new
+targets grouped with the same platform or product family in either production/ or
+development/.
+
+## Validation Expectations
+
+* Keep topology2 buildable through the topologies2 target
+* Preserve compatibility with alsatplg pre-processing mode used by the build system
+* Ensure topology edits remain synchronized with nearby architecture or README documentation when design behavior changes