fix(opencode): add DeepSeek system prompt

[punkisnotdead3]

Issue for this PR

Closes #38234

Type of change

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

What does this PR do?

DeepSeek models currently inherit the generic system prompt, including conflicting instructions that encourage extremely short replies, one-word answers, stopping immediately after an edit, and delegating routine exploration to Task.

This adds a dedicated DeepSeek system prompt with one consistent workflow. It prefers direct read/grep/glob exploration, reserves Task for genuinely independent work, and routes direct, aliased, V4, and provider-prefixed DeepSeek model IDs to that prompt.

How did you verify your code works?

• bun test test/session/system.test.ts
• bun typecheck
• bun run lint packages/opencode/src/session/system.ts packages/opencode/test/session/system.test.ts
• bunx prettier --check packages/opencode/src/session/system.ts packages/opencode/test/session/system.test.ts
• Push hook: repository-wide Turbo typecheck (30/30 packages passed)

Screenshots / recordings

N/A — no UI changes.

Checklist

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

[github-actions]

Thanks for your contribution!

This PR doesn't have a linked issue. All PRs must reference an existing issue.

Please:

1. Open an issue describing the bug/feature (if one doesn't exist)
2. Add Fixes #<number> or Closes #<number> to this PR description

See CONTRIBUTING.md for details.

[github-actions]

The following comment was made by an LLM, it may be inaccurate:

I found one potentially related PR that's worth noting:

PR #31867: feat: improve deepseek prompt cache reuse
#31867

This PR is related to DeepSeek optimization (prompt cache reuse), though it appears to address a different aspect than the current PR. The current PR (38229) is specifically about adding a dedicated DeepSeek system prompt with consistent workflow instructions, while #31867 focuses on cache reuse optimization.

These are complementary improvements to DeepSeek support rather than duplicates. The current PR is a newer, more targeted fix for system prompt behavior.

[github-actions]

Thanks for updating your PR! It now meets our contributing guidelines. 👍

[github-actions]

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[punkisnotdead3]

Hi maintainers — this PR is still actively maintained. I resolved the reported conflicts against dev in d12ca0e and verified the targeted tests, package typecheck, and repository-wide pre-push typecheck. Could a maintainer please reopen it for review? Thanks!