fix(discover): match Windows project slugs case-insensitively

[w3lld1]

Summary

• Fixes Windows rtk discover current-project matching when Claude Code stores project slugs with a different drive-letter/path case.
• Updates Windows path slug encoding to replace : with -, matching the reported Claude Code e--... project directory shape.
• Adds a regression test with uppercase E:\... cwd input matching a lowercase e--... Claude project directory.

Fixes #2919

Test plan

• cargo fmt --all --check
• cargo clippy --all-targets
• cargo test
• cargo test discover::provider::tests::test_encode_project_path_windows
• cargo test discover::provider::tests::test_discover_sessions_matches_windows_project_filter_case_insensitively

I could not manually reproduce on Windows from this Linux environment; the regression test simulates the reported project-directory casing/slug mismatch.

[CLAassistant]

All committers have signed the CLA.

[KuSh]

Closing this as a duplicate of #2931, which makes the same project-directory match case-insensitive but keeps it to Windows via cfg!(windows), so Unix matching is unchanged, and which merges cleanly against develop. The colon half of this PR already landed on develop through #2952. Please take a look at #2931 and comment there, or reply here if you think the unconditional match was deliberate.