rebrand: remove confusing DeepSeek wording from CodeWhale-first surfaces

[Hmbown]

Problem

CodeWhale still intentionally ships deepseek / deepseek-tui legacy shim binaries during v0.8.x, and DeepSeek remains a first-class provider. But user-facing release/install/help text can still make it look like the project itself is still named DeepSeek.

Goal

Audit the public CodeWhale-first surfaces and make the naming distinction crisp: CodeWhale is the product, DeepSeek is a provider and temporary legacy shim namespace.

Acceptance criteria

• Release notes, npm README, install docs, CLI help, migration docs, and update messaging use CodeWhale as the product name.
• deepseek / deepseek-tui are mentioned only as legacy compatibility shims or as provider/model support where that is actually meant.
• The docs still reassure existing users that DeepSeek provider support is not being removed.
• No change republishes the deprecated deepseek-tui npm package.
• Add or update tests/snapshots where CLI help or migration text is covered.