Normalize custom base URL by trimming trailing slash in init

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Task summary

Normalize custom provider base URLs entered in commit-echo init to avoid double slashes in downstream endpoint construction (for example, .../v1/ + /chat/completions).

Why this is a good first issue

The change is small and localized to setup/config handling plus a focused regression test.

Suggested files or areas

• src/commands/init.ts
• Possibly provider request path construction tests

Acceptance criteria

• Trim trailing slash(es) from custom base URL input before saving config.
• Preserve valid URL semantics.
• Add a test proving normalized URL behavior.
• Ensure existing provider tests continue to pass.

Extra context

Some APIs tolerate duplicate slashes, but normalizing early reduces subtle integration bugs.