Capture shared codegen timestamp instead of empty run_started_at

[ptr727]

Problem

The dual-target codegen "byte-identical output" mechanism (#92) was silently broken. run-codegen-pull-request-task.yml passed --runtime "${{ github.run_started_at }}" to both matrix legs, but github.run_started_at resolves to an empty string in this reusable-workflow context. CodeGenBuilder falls back to DateTime.UtcNow when --runtime is empty, so each leg stamped its own wall-clock time — the main and develop legs diverged (observed ~5s apart), and CodeGen.cs differed between branches, reintroducing the exact develop → main release conflict the mechanism was meant to prevent.

Fix

Capture one UTC timestamp in a new get-runtime job and feed it to both matrix legs via needs.get-runtime.outputs.runtime. Both legs now receive an identical, non-empty value, so CodeGen.cs is byte-identical across branches again.

Discovered while resolving the CodeGen.cs conflict on the develop→main release PR (#99). AGENTS.md updated to match.

🤖 Generated with Claude Code

[Copilot]

Pull request overview

This PR restores deterministic, shared timestamps for the dual-target codegen workflow so the main and develop matrix legs generate byte-identical CodeGen/CodeGen.cs output, preventing recurring develop → main release merge conflicts.

Changes:

• Adds a get-runtime job to capture a single UTC timestamp once per workflow invocation and exposes it as a job output.
• Updates the codegen matrix job to use the shared timestamp via needs.get-runtime.outputs.runtime (instead of github.run_started_at, which is empty in this reusable-workflow context).
• Updates AGENTS.md documentation to reflect the new mechanism and the reusable-workflow github.run_started_at pitfall.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
AGENTS.md	Updates the branching/codegen guidance to document the shared runtime capture approach and why github.run_started_at cannot be used here.
.github/workflows/run-codegen-pull-request-task.yml	Introduces get-runtime and wires its output into both matrix legs so --runtime is always non-empty and identical across legs.