fix: encode Windows CWD paths for web routes

[creamtea47]

Summary

• encode Windows backslashes in Codex CWD project identifiers
• keep generated web session links to one route segment
• cover a Windows CWD in the existing TestEncodePath table

Validation

• go test ./internal/parser -run '^TestEncodePath$'
• opened a real Windows Codex session at the generated route and received HTTP 200

[lroolle]

Verified: go test ./internal/parser green in CI, and the change is the minimal one. DecodePath stays lossy for Windows paths, but it already was for / (see the note in internal/provider/multi.go), and the route now resolves. Thanks for the fix and the test case.