[WIP] fix: add funder and signature_type to examples

[vikions]

Overview

Examples order.py and market_buy_order.py are missing funder and signature_type parameters, causing errors for most users. This PR updates examples to match README documentation.

Description

The current examples don't include critical parameters that are documented in README:

• Added funder parameter (address holding funds on Polymarket)
• Added signature_type parameter (0=EOA, 1=Magic, 2=Proxy)
• Updated .env.example with new variables and explanations
• Changed examples from AMOY testnet to POLYGON mainnet to match README

Real-world experience:
While building Opipolix Bot (a Polymarket/Opiniona trading bot), we experienced invalid signature and not enough balance / allowance errors when following the examples. The bot only started working after adding funder and signature_type parameters — exactly as documented in README but missing from examples.

Our working implementation:

signature_type = 2 if self.funder_address else 0

self.client = ClobClient(
    host=CLOB_URL,
    key=self.private_key,
    chain_id=CHAIN_ID,
    signature_type=signature_type,
    funder=self.funder_address,
)

Related issues: #109, #198, #187

Testing instructions

No changes to test runner. Examples can be tested manually with valid credentials.

Types of changes

• Bug fix/behavior correction

Notes

README was updated ~3 months ago with correct parameters, but examples (~8 months old) were not updated to match.

Status

• Prefix PR title with [WIP] if necessary (changes not yet made).
• Add tests to cover changes as needed.
• Update documentation/changelog as needed.
• Verify all tests run correctly in CI and pass.
• Ready for review/merge.

---

Note

Examples updated to match current client usage and mainnet defaults

• Add funder and signature_type to ClobClient initialization in order.py and market_buy_order.py; remove manual ApiCreds wiring and instead create_or_derive_api_creds() + set_api_creds()
• Switch network constant from AMOY to POLYGON; default host remains https://clob.polymarket.com
• Adjust example order params/comments (e.g., price 0.50, clarify BUY amount is USDC; use OrderType.FOK for market buy)
• Expand .env.example with FUNDER, SIGNATURE_TYPE, and clearer API URL/credential guidance

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