feat: allow overriding system prompt date via OPENCODE_DATE env var

[nilo85]

Enables deterministic LLM request replay in CI integration tests by fixing the date string in the system prompt.

Issue for this PR

Type of change

• Bug fix
• New feature
• Refactor / code improvement
• Documentation

What does this PR do?

The system prompt in session/system.ts includes a dynamic Today's date: ${new Date().toDateString()} line. This makes it impossible to match recorded LLM requests when replaying them in CI integration tests, since the date changes every day.

This PR adds an OPENCODE_DATE environment variable that overrides the date string when set, falling back to the real date otherwise. It follows the established RuntimeFlags pattern used for other runtime overrides in the project.

Usage:
OPENCODE_DATE="Thu Jan 01 2025" opencode

How did you verify your code works?

I built it locally, and ran:

OPENCODE_DATE="Yesterday" ./result/bin/opencode

Then I verified the system promt in the capture of llama-swap (see comment)

Screenshots / recordings

N/A — no UI changes.

Checklist

• I have tested my changes locally
• I have not included unrelated changes in this PR

[nilo85]

OPENCODE_DATE="Yesterday" ./result/bin/opencode

[github-actions]

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[github-actions]

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