release: bump version to v0.39.0

[emil14]

Summary

• bump repository version references from 0.38.0 to 0.39.0
• update checked-in Neva manifests, docs, test expectations, generated version header, and pkg.Version
• teach the release-neva skill to do repo-wide version bumps during release prep

Release basis

• User-facing stdlib additions since v0.38.0: std/io explicit file handles and expanded std/os env/process/fs surface
• Parser fix: missing connection sides now return compiler errors instead of panicking
• Internal harness consolidation is included but not the main release reason

Validation

• rg "v?0\.38\.0" --glob "!neva-*" --glob "!*.exe" -> no matches
• git diff --check origin/main...HEAD
• go test ./internal/builder/tests ./internal/compiler/analyzer ./internal/compiler/desugarer ./pkg ./internal/cli
• make build
• ./neva-darwin-arm64 version -> 0.39.0
• direct local golangci-lint run ... cannot load config because installed golangci-lint was built with Go 1.25 while this repo targets Go 1.26.4
• previous pre-push: gofix-check, test-unit, and govulncheck passed; lint failed on existing internal/compiler/typesystem/* findings unrelated to this release bump

Local release asset checksums

edec800be80375cdcc31382890487428ea835bfb24c6f83d41d6581532d87d90  ./neva-darwin-amd64
ebca5c58d9c42bf6eb502cfaf073bdb7b2b1e81f9f6b8cce36789a287148a11e  ./neva-darwin-arm64
211d9a943828b46cf6d2efba7c138f19d25981686bc041c4241df83c85ec6922  ./neva-linux-amd64
fd6fecca4c56db9dac9175c386fef324cc532e8691008aa490e04407de685ee4  ./neva-linux-arm64
50e444a60c9cb49717824db256e1aae9a94d090358d48423e861d3f2765e33bc  ./neva-linux-loong64
c5fbbe8c49a9cabd2182a7d9212c1f2329fc2a4320435daac6f9a301442efdbd  ./neva-windows-amd64.exe
6581efb233b96ec675ff8e97bc8e4dd0eb81a8ff16ab3a02fabb68fe3d67e8a2  ./neva-windows-arm64.exe