chore: migrate to AGENTS.md convention and add specialized agents

[Copilot]

Summary

Migrates Copilot instructions from .github/instructions/*.instructions.md to standard AGENTS.md convention. Adds specialized agents for documentation, linting and testing. See the VSCode docs for more detail.

Changes

Migration

• .github/instructions/source.instructions.md → /AGENTS.md (root-level, source code focus)
• .github/instructions/models.instructions.md → /examples/AGENTS.md (examples focus)

Additions

• New Copilot agents, following the GitHub guidelines.

Changes

• Updated CONTRIBUTING.md to reference new structure and explain AGENTS.md convention.
• Added chat.useNestedAgentsMdFiles setting to devcontainer.

[github-actions]

Benchmark comparison for 9f17f083 (base) vs 02797e8b (PR)

------------------------------------------------------------------------------------------------------------------ benchmark: 2 tests ------------------------------------------------------------------------------------------------------------------
Name (time in ms)                                                                         Min                 Max                Mean            StdDev              Median                IQR            Outliers     OPS            Rounds  Iterations
--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
test_benchmark_process_run (main/.benchmarks/Linux-CPython-3.12-64bit/0001_base)     456.7513 (1.0)      468.4316 (1.0)      462.0195 (1.0)      4.6218 (1.0)      460.6004 (1.0)       6.9625 (1.0)           2;0  2.1644 (1.0)           5           1
test_benchmark_process_run (pr/.benchmarks/Linux-CPython-3.12-64bit/0001_pr)         467.2946 (1.02)     481.2752 (1.03)     473.1326 (1.02)     6.8958 (1.49)     469.9046 (1.02)     12.9211 (1.86)          1;0  2.1136 (0.98)          5           1
--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------

Legend:
  Outliers: 1 Standard Deviation from Mean; 1.5 IQR (InterQuartile Range) from 1st Quartile and 3rd Quartile.
  OPS: Operations Per Second, computed as 1 / Mean

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

📢 Thoughts on this report? Let us know!

[github-actions]

Benchmark comparison for 9f17f083 (base) vs 3d4e68eb (PR)

------------------------------------------------------------------------------------------------------------------ benchmark: 2 tests -----------------------------------------------------------------------------------------------------------------
Name (time in ms)                                                                         Min                 Max                Mean            StdDev              Median               IQR            Outliers     OPS            Rounds  Iterations
-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
test_benchmark_process_run (pr/.benchmarks/Linux-CPython-3.12-64bit/0001_pr)         448.0453 (1.0)      460.2226 (1.00)     451.6902 (1.0)      4.8504 (1.59)     450.0620 (1.0)      3.2963 (1.20)          1;1  2.2139 (1.0)           5           1
test_benchmark_process_run (main/.benchmarks/Linux-CPython-3.12-64bit/0001_base)     450.2647 (1.00)     458.6723 (1.0)      453.8027 (1.00)     3.0537 (1.0)      453.0938 (1.01)     2.7498 (1.0)           2;0  2.2036 (1.00)          5           1
-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------

Legend:
  Outliers: 1 Standard Deviation from Mean; 1.5 IQR (InterQuartile Range) from 1st Quartile and 3rd Quartile.
  OPS: Operations Per Second, computed as 1 / Mean