Stop reading from local config directory.

[fnando]

What

Stop reading from local config directory. We're still checking for .stellar directories and warning about the required migration.

Why

To complete the removal from v26. Close #2493

Known limitations

N/A

[Copilot]

Pull request overview

This PR completes the v26 removal of reading deprecated local config directories (e.g. .stellar), while still detecting their presence and warning users to migrate—addressing the --config-dir ancestor-search secret disclosure described in issue #2493.

Changes:

• Make local-config discovery use the real process cwd (so --config-dir no longer seeds ancestor search).
• Stop reading/listing identities and contract aliases from local config, but emit deprecation/migration warnings when local config is detected.
• Update soroban-test integration helpers/tests to write/read identities from the global config directory layout.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 3 comments.

File	Description
cmd/soroban-cli/src/config/locator.rs	Avoids --config-dir ancestor-walk for local discovery; skips local reads/lists while warning; adds regression tests.
cmd/crates/soroban-test/tests/it/util.rs	Updates test helper to write identities directly into the global config dir structure.
cmd/crates/soroban-test/tests/it/config.rs	Adjusts integration tests to use sandbox.config_dir() paths and global-layout expectations.