feat: add DeepSeek TUI harness support

[EmiyaKiritsugu3]

What problem are you trying to solve?

DeepSeek TUI (Hmbown/DeepSeek-TUI) already auto-discovers superpowers skills from the workspace skills/ directory and lists them in the ## Skills section of the system prompt. However, the using-superpowers bootstrap -- the mandate that forces the model to check for relevant skills before taking any action -- was never injected into the context at session start.

Without the bootstrap, skills are passive metadata: the model sees them listed but has no reason to invoke them. The acceptance test ("Let's make a react todo list" triggers brainstorming) fails.

What does this PR change?

Adds DeepSeek TUI as a supported harness, matching the existing pattern for Gemini CLI (GEMINI.md), OpenCode (plugin), and Claude Code (hooks).

• AGENTS.md: Replaced the symlink-to-CLAUDE.md with a standalone file that prepends the using-superpowers bootstrap before the existing contributor guidelines. DeepSeek TUI loads AGENTS.md as project_instructions at session start, so the bootstrap is now auto-injected.
• DEEPSEEK.md: New file following the GEMINI.md @include pattern, for when deepseek-tui adds native DEEPSEEK.md support (PR: feat: add DEEPSEEK.md as project context file Hmbown/CodeWhale#1852).
• references/deepseek-tools.md: Tool mapping for DeepSeek TUI equivalents (TodoWrite to checklist_write, Task to agent_open, Skill to load_skill, etc.).
• SKILL.md: Added DeepSeek TUI to How to Access Skills and Platform Adaptation sections.
• README.md: Added DeepSeek TUI to the Quickstart list and installation section.

Is this change appropriate for the core library?

Yes. This adds support for a new harness (IDE/CLI tool), which is explicitly listed as acceptable in the contributor guidelines. No third-party dependencies are introduced.

What alternatives did you consider?

1. Modify only AGENTS.md -- but AGENTS.md was a symlink to CLAUDE.md. Breaking the symlink was unavoidable.
2. Wait for deepseek-tui native DEEPSEEK.md support -- the upstream PR (feat: add DEEPSEEK.md as project context file Hmbown/CodeWhale#1852) is open, but the integration works today via AGENTS.md without it.
3. User-level instructions.md -- the ~/.deepseek/instructions.md file is auto-generated and can be overwritten; unreliable as a bootstrap anchor.

Does this PR contain multiple unrelated changes?

No. All changes serve the same goal: adding DeepSeek TUI harness support.

Existing PRs

• I have reviewed all open AND closed PRs for duplicates or prior art
• Related PRs: none found

Environment tested

Harness	Harness version	Model	Model version/ID
DeepSeek TUI	0.8.39	deepseek-v4-pro	latest

New harness support

Acceptance test performed: the user ran /skill using-superpowers inside a DeepSeek TUI session in this workspace. The full SKILL.md content loaded into context, including the mandate ("IF A SKILL APPLIES, YOU MUST USE IT"), Red Flags, and skill flow diagram.

The bootstrap is also auto-loaded from AGENTS.md as project_instructions at every session start -- no manual /skill invocation needed for normal use.

Evaluation

• Initial prompt: "Vamos adicionar suporte ao deepseek-tui do superpowers"
• Before: skills listed but no bootstrap -- model had no reason to invoke them
• After: AGENTS.md with bootstrap auto-loaded -- bootstrap mandate is present in project_instructions

Rigor

• This change was tested adversarially, not just on the happy path
• I did not modify carefully-tuned content (Red Flags table, rationalizations, "human partner" language) without extensive evals showing the change is an improvement

Note: No existing skills were modified in content -- only new platform references were added.

Human review

• A human has reviewed the COMPLETE proposed diff before submission

[EmiyaKiritsugu3]

Closes #1587

[obra]

Hi, This looks like it was not really tested or designed.
I'd love deepseek tui support. But we need to use their plugin install mechanism, this would need to target the dev branch, not main, and you'd need to not turn AGENTS.md into a file instead of the symlink it is today.