[codex] Use dynamic Stellar fee bumps

[Dreamstore2046]

Summary

• Add shared/stellar-fee.ts with Horizon /fee_stats p90 targeting, capped fallback behavior, and tx_insufficient_fee detection.
• Submit bill payments and setup trustlines through signed Stellar fee-bump envelopes, doubling the inner fee for up to 3 bumps.
• Expose stellar_fee_bumps_total and add regression tests for fee selection, fee-bump submission, and metrics output.

Notes

• The issue requested stellar.fee.bumps_total; Prometheus metric names do not allow dots, so this exports the conventional stellar_fee_bumps_total name.

Validation

• npm test -- shared/__tests__/stellar-fee.test.ts shared/__tests__/metrics.test.ts agent/__tests__/dynamic-fee.test.ts
• npm test -- scripts/__tests__/setup-wallets.test.ts
• npx tsc --noEmit --target ES2022 --module ESNext --moduleResolution bundler --allowImportingTsExtensions --esModuleInterop --allowSyntheticDefaultImports --strict --skipLibCheck --types node,vitest shared/stellar-fee.ts shared/__tests__/stellar-fee.test.ts shared/metrics.ts shared/__tests__/metrics.test.ts agent/__tests__/dynamic-fee.test.ts scripts/setup-wallets.ts
• git diff --cached --check

Known existing issue: including agent/tools.ts in targeted typecheck still hits the pre-existing x402 scheme template literal errors at agent/tools.ts:160 and agent/tools.ts:162.

Closes #22

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