feat(init): add Whale agent support

Hi, RTK is a great. I really like that it focuses on reducing noisy shell output while keeping the underlying commands explicit and inspectable.

I maintain/work on Whale (https://github.com/usewhale/DeepSeek-Code-Whale), a DeepSeek-native coding agent CLI/TUI, and would be happy to submit a PR for RTK support.

Whale (https://github.com/usewhale/DeepSeek-Code-Whale) has a PreToolUse hook for shell_run that can rewrite tool input via updated_input, so RTK can integrate as a transparent full-hook adapter rather than prompt-only rules.

Proposed scope:

• add rtk init --agent whale
• write a Whale [[hooks.PreToolUse]] config for shell_run
• add rtk hook whale to read Whale's hook JSON from stdin and return updated_input
• fail open if RTK is missing, input is invalid, or the command should not be rewritten
• add tests and docs

Before I start the PR: would you prefer this as a first-class supported agent in RTK?

Whale Hooks: https://github.com/usewhale/DeepSeek-Code-Whale/blob/main/docs/hooks.en.md