Add "pi" to the Kimi Code approved coding-agent whitelist

[da-moon]

Request: whitelist the pi coding agent on api.kimi.com/coding, so pi
users can use Kimi Coding membership without impersonating another client.

Context

• pi is an open-source terminal coding agent (https://pi.dev). Users want
  to drive Kimi Coding (kimi-for-coding / K2.7 Code) from pi using their
  Kimi membership.
• api.kimi.com/coding admits requests only from approved clients by
  inspecting User-Agent. Today pi cannot use Kimi Coding without
  spoofing an approved client identity (for example KimiCLI/*).
• Spoofing is prohibited by the Kimi Code Community Guidelines ("Don't
  spoof or alter client identity information"), so we will not ship that.
  We want our own identifier whitelisted instead.

Request

• Whitelist the pi/<version> User-Agent (and X-Msh-Platform: pi) on
  api.kimi.com/coding, on the same terms as Kimi CLI, Claude Code,
  Roo Code, and Kilo Code.
• pi sends its own stable identifier. This is not a request to spoof any
  other client.

Why this is small

• The whitelist is prefix-based (KimiCLI/ and claude-code/ both pass
  today), so a single allowlist entry covers pi.
• Endpoint, model id, and the OAuth device flow are unchanged; pi already
  implements the public device-code flow against auth.kimi.com.

Precedent

• [Feature Request] Add Cline to the Kimi Code whitelist of supported coding agents kimi-cli#2322 (Cline)
• Add Zoo Code to the third-party coding agent API whitelist kimi-cli#2416 (Zoo Code)

References

• Community Guidelines:
  https://www.kimi.com/code/docs/en/kimi-code/community-guidelines.html
• Third-party agents:
  https://www.kimi.com/help/kimi-code/third-party-agents