fix(stellar): unify agent secret env var and correct futurenet passphrase

[manoahLinks]

Summary

• Standardize on STELLAR_AGENT_SECRET_KEY as the single env var name across config, client, tests, docs, and .env.example — eliminates silent runtime failures from mismatched variable names
• Fix getNetworkPassphrase() to explicitly map futurenet → Networks.FUTURENET instead of falling through to TESTNET; throws a clear error for unknown network values
• Add unit tests covering the full network passphrase mapping (mainnet, testnet, futurenet, unknown, undefined)

Test plan

• getAgentKeypair() succeeds when only STELLAR_AGENT_SECRET_KEY is set
• No remaining references to STELLAR_AGENT_SECRET or AGENT_SECRET_KEY
• getNetworkPassphrase() returns correct passphrase for futurenet
• Testnet passphrase no longer applies to futurenet
• All 19 stellar client tests pass
• npm run lint passes

Closes #29
Closes #31

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