PluralKit protocol — handle proxied messages

[transfaeries]

Summary

• Detect webhook-proxied messages (PluralKit, Tupperbox, etc.) and prevent double-replies
• 2-second wait-before-reply lets proxy bots send their webhook copy before faebot generates
• History swap replaces original message entry with the proxy version (correct author name)
• Content matching handles both tag-stripping (substring) and autoproxy (exact match)
• 18 new tests covering detection, matching, history swap, and full async flow

Closes #13

Test plan

• All 61 tests pass
• Deploy to fly.io and live-test with PK messages in #live-testing
• Verify single reply to proxied messages (no double-reply)
• Verify conversation history shows proxy member name (e.g. "Minou | transfaeries")
• Verify non-PK messages still work normally (with 2s added latency)
• Verify admin commands work while autoproxying

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