Ethiopia-specific calibration sources

[SeaCelo]

Update Ethiopia-specific calibration sources.

alpha_T — the IMF GFS series is zero for Ethiopia, which is a reporting gap not a real zero. The default in ogeth_default_parameters.json now reflects the FY2024/25 IMF program target for PSNP/UPSNP cash transfers.

alpha_G — switches from IMF GFS to the World Bank general government final consumption series. The IMF budgetary central government view understates government consumption by ~4pp because Ethiopia's regional states do substantial spending outside it.

g_y_annual — trims the growth averaging window to 2006 onward, excluding the pre-reform period.

r_gov_shift / r_gov_scale — removed from live calibration. The OLS uses only hardcoded inputs and always produces the same result. The values stay in the defaults JSON and the docs keep the methodology with a reproducible snippet.

Documentation (macro.md, exogenous_parameters.md)

• PSNP sourcing for alpha_T
• World Bank sourcing for alpha_G
• Updated values in the exogenous parameters table
• Reproducible snippet for r_gov_shift/r_gov_scale
• FY2023/24 MoF references for debt and foreign-debt notes

[SeaCelo]

Updates calibration values and other fixes. Should be merged after #2.

@jdebacker @rickecon