2026-08-01

Layer `.bucksettings.local.toml` in home dir over repo root settings

Summary:
Parses `.bucksettings.local.toml` from home dir. Home-local settings are recursively merged over the repo-root `.bucksettings.toml`. The pipeline is now discover settings files -> parse into tables -> merge into one table -> deserialize into `BuckSettingsData`.

Adds unit coverage for layered parsing and an e2e test verifying that a home-local value overrides the repo value.

Reviewed By: Nero5023

Differential Revision: D113876491

fbshipit-source-id: d04ffa478d03070b0e4a6619140b7fc9e5dc67a5