feat: add DEEPSEEK.md as project context file

[EmiyaKiritsugu3]

Adds DEEPSEEK.md to PROJECT_CONTEXT_FILES so harnesses like superpowers can ship a harness-specific instruction file that gets auto-loaded as project_instructions — analogous to GEMINI.md in Gemini CLI.

What

1 line in the PROJECT_CONTEXT_FILES constant, positioned after AGENTS.md so harness-specific files take priority over generic CLAUDE.md while universal AGENTS.md remains top.

Files changed

• project_context.rs: Add DEEPSEEK.md to PROJECT_CONTEXT_FILES and module docstring
• working_set.rs: Add DEEPSEEK.md to key-file detection candidates
• utils.rs: Add deepseek.md to fallback key-file matcher

Why

Superpowers needs a harness-specific bootstrap file to inject the using-superpowers skill at session start. AGENTS.md is already used for universal contributor guidelines and shouldn't be repurposed. Prior art: Gemini CLI reads GEMINI.md, OpenCode reads .opencode/plugins/.

Closes #1851

[EmiyaKiritsugu3]

Note: Rust toolchain (cargo) is not available in my local environment, so I could not run cargo fmt --check, cargo clippy, or cargo test before submitting.

However, the change is trivially verifiable by inspection:

1. project_context.rs: Adds "DEEPSEEK.md" to a static array constant and one docstring line — cannot break compilation
2. working_set.rs: Adds "DEEPSEEK.md" to a static array constant — same
3. utils.rs: Adds "deepseek.md" to a pattern match — same

I will address any CI failures reported by GitHub Actions.

[gemini-code-assist]

Code Review

This pull request adds support for DEEPSEEK.md as a project context and key file across the TUI crates. The changes include updating the file priority list, key file detection logic, and documentation. Feedback suggests reordering the priority list in project_context.rs to place DEEPSEEK.md above AGENTS.md, ensuring that tool-specific instructions are prioritized over universal ones.

[gemini-code-assist]

The current priority order places AGENTS.md above DEEPSEEK.md. Since load_project_context stops at the first match it finds in the PROJECT_CONTEXT_FILES list, any project containing a universal AGENTS.md will ignore the harness-specific DEEPSEEK.md. This appears to conflict with the PR's stated goal of providing a bootstrap file that doesn't require repurposing AGENTS.md for tool-specific instructions.

To allow tool-specific overrides while still falling back to universal instructions when the specific file is missing, DEEPSEEK.md should be placed at the top of the list. You should also update the module docstring to reflect this priority change.

Suggested change

	"AGENTS.md",
	"DEEPSEEK.md",
	"DEEPSEEK.md",
	"AGENTS.md",

[EmiyaKiritsugu3]

Thanks for the review. Here's the rationale for keeping AGENTS.md at higher priority than DEEPSEEK.md:

AGENTS.md is the universal project-instructions file recognized by multiple agents (Claude Code, Gemini CLI, Cursor, and now DeepSeek TUI). Placing a harness-specific file above it would mean that in any project that ships DEEPSEEK.md, the AGENTS.md instructions (which may contain build commands, conventions, architecture notes, or security policies shared across all agents) would be silently shadowed unless the user explicitly maintains both files in sync.

By keeping AGENTS.md first:

1. Existing AGENTS.md setups are unaffected -- the addition of DEEPSEEK.md is purely additive, never overriding.
2. Users who want DeepSeek TUI-specific overrides add DEEPSEEK.md and can reference AGENTS.md from it if needed.
3. The pattern matches how CLAUDE.md (generic) already sits below AGENTS.md (universal) in the same priority list.

DEEPSEEK.md is designed as a complementary file -- analogous to how CLAUDE.md complements AGENTS.md -- not a replacement for it.