Allow prefixed workspace config files

[rafael81]

Summary

• allow workspace config discovery to match *workspace*.toml, including the funded xx-workspace.toml use case from Stub issue for bug bounty #1 / Allow file sorting of the workspace.toml files orf/git-workspace#207
• keep excluding workspace-lock.toml
• document prefixed config filenames in the README
• add unit coverage for prefixed config discovery and lockfile exclusion

Fixes #1.

Validation

• cargo fmt -- --check
• git diff --check
• cargo test attempted, but dependency build failed before compiling this crate: openssl-sys v0.9.58 cannot preprocess Homebrew OpenSSL 3 headers on this machine.

Transparent note: this PR was prepared with AI assistance, with manual review of the issue, diff, and validation output.

Funded issue

• IssueHunt $4 issue: https://oss.issuehunt.io/r/Kreyren/git-workspace/issues/1

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IssueHunt Summary

Referenced issues

This pull request has been submitted to:

• #1: Stub issue for bug bounty

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