Adopt upstream #2952: discover - sanitize drive-letter colon so Windows discover finds sessions #25

Adopt rtk-ai/rtk#2952 by @zbl1998-sdjn. Batch 2 (2026-07-19), Windows tier.

Windows rtk discover scans 0 sessions: missing : in SANITIZED_CHARS means the project-path slug never matches Claude's project dirs. 1-char root cause, TDD, Windows-verified upstream. Duplicates to reference/close: rtk-ai#3007, rtk-ai#2368, rtk-ai#3043; related case-insensitivity half of rtk-ai#2931 and rtk-ai#2920.

Adoption checklist (per CONTEXT.md)

• Sync: git merge upstream/develop into develop
• Repro on pre-cherry-pick binary
• Cherry-pick preserving authorship; confirm fixed behavior with real output in the PR
• Quality gate: cargo fmt --all && cargo clippy --all-targets && cargo test --all
• Amendments (if needed) as separate fork-authored commits