[config] fork.toml lowered profit gate has no mainnet-accidental-use guard

[obchain]

Refs #52

File: config/fork.toml

Problem

fork.toml lowers profit threshold for demo (per PR description). If operator accidentally runs charon --config config/fork.toml with mainnet env vars (BNB_WS_URL pointing at mainnet), bot operates on mainnet with lowered profit gate. Unprofitable liquidations attempted.

Fix

Add runtime assertion in main.rs that refuses to start if:

• config rpc_url ≠ localhost AND config profile_name = 'fork'
• OR config profile_name contains 'fork' AND chain_id matches mainnet

Add profile_tag = "fork" field to fork.toml. Validate on startup.

[obchain]

Fixed on feat/24-anvil-fork (fe60699): Added bot.profile_tag: Option<String> to BotConfig, set profile_tag = "fork" in config/fork.toml, and introduced Config::validate() with a typed ConfigError::ForkProfileNonLoopbackRpc { chain, field, url } variant. When the fork profile points any chain's ws_url/http_url at a non-loopback host, validation fails with the exact actionable copy from the ticket. crates/charon-cli/src/main.rs calls validate right after Config::load and exits non-zero on failure, so a lowered profit gate can never land against a mainnet RPC. Non-fork profiles are unrestricted — local-geth dev runs still work.