fix: add env fallbacks for hardcoded RPC endpoints

[realfishsam]

Fixes #507

[realfishsam]

PR Review: PASS (NOT VERIFIED)

What This Does

Extracts hardcoded RPC endpoint URLs into environment-variable-backed constants for Limitless (Base chain) and Opinion (BSC chain). Creates a new config.ts for Limitless and updates the existing Opinion config.ts to read from process.env.LIMITLESS_RPC_URL and process.env.OPINION_RPC_URL respectively, with the original URLs as defaults.

Blast Radius

• Limitless exchange: client.ts (getBalance), index.ts (getAddressOnChainBalance) -- both RPC call sites updated
• Opinion exchange: config.ts -- one constant updated, consumed by auth.ts
• No SDK, OpenAPI, or normalizer changes

Findings

1. Consistent approach across both exchanges -- both follow the process.env.X || 'default' pattern. Good.
2. New file limitless/config.ts is a single-line export, which is fine and follows the Opinion config pattern.
3. No test coverage for env var override -- not blocking since the default behavior is preserved and the fallback is the existing hardcoded URL, but a unit test verifying process.env override would strengthen confidence.
4. Other hardcoded RPC URLs in codebase -- grep shows no remaining un-configurable RPC URLs after this PR. Clean sweep.

PMXT Pipeline Check

• Field propagation: N/A
• OpenAPI sync: N/A
• Type safety: OK

Semver Impact

patch -- no public API change; internal infrastructure improvement for deployment flexibility

Risk

No runtime verification that the env var override actually works end-to-end (e.g., connecting to a custom RPC). The change is safe by construction since the default is unchanged.