[core] TOML config loader with ${ENV_VAR} substitution

[obchain]

Context

Bot-level knobs, per-chain RPC endpoints, per-protocol addresses, and flash-loan sources all live in a TOML file. Secrets (RPC URLs) are referenced via ${ENV_VAR} placeholders and substituted from the environment at load time — no secrets ever in the repo.

Scope

• Config struct hierarchy: bot, chain, protocol, flashloan, liquidator
• Hash-map keyed sections so adding chains / protocols later is a config edit, not a schema change
• Config::load(path) — reads → substitutes env vars → parses via serde
• Hand-rolled substitute_env_vars() helper that errors cleanly on unset vars
• config/default.toml with real BSC addresses (Venus comptroller, Aave V3 Pool)
• .env.example documenting BNB_WS_URL and BNB_HTTP_URL

Acceptance criteria

• Config::load(path) returns anyhow::Result<Config>
• Env substitution works for string values inside TOML
• Parse errors include the file path for context
• config/default.toml parses with a populated .env (copied from .env.example)

References

• Delivered in commit 2495292
• PRD section 4d (Config Loader)

[obchain]

Superseded — TOML config loader shipped in feat/04-toml-config-loader.