Honour --config-dir when loading/saving defaults.

[fnando]

What

Honour --config-dir when loading/saving defaults.

Why

• Close https://github.com/stellar/stellar-cli-internal/issues/28
• Close https://github.com/stellar/stellar-cli-internal/issues/29

Known limitations

N/A

[Copilot]

Pull request overview

This PR ensures the CLI honors --config-dir when reading and writing config.toml defaults (network/identity/inclusion fee), aligning startup behavior and “use” commands with the selected config profile.

Changes:

• Add path-aware Config::load / Config::save_to helpers and route new()/save() through them.
• Update default write/unset helpers to read/write config.toml under the active config dir.
• Update startup env-default loading to prefer --config-dir (parsed from raw args) and add integration tests covering both startup reads and default writes.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 1 comment.

File	Description
cmd/soroban-cli/src/config/mod.rs	Introduces path-based load/save helpers to support non-global config locations.
cmd/soroban-cli/src/config/locator.rs	Writes/unsets defaults using a config.toml path derived from the active config dir.
cmd/soroban-cli/src/cli.rs	Startup defaults loading now honors --config-dir by scanning raw CLI args before clap parsing.
cmd/crates/soroban-test/tests/it/config.rs	Adds integration tests ensuring --config-dir affects startup defaults and where defaults are written.