Derive the manifest's versioned paths from id and version at build time

[bennett-elder]

Fixes #10.

manifest.json carries two paths shaped <id>/<version>/… — backend.assemblyDir and each skills[].folder. The host stages the bundle to <ExtensionStudioPath>/<id>/<version>/, so both move every time version is bumped, and nothing kept them in sync.

build-extension.sh now derives both when it writes the bundle manifest, in the same jq call that pins sdkVersion. It prints a NOTE: per field when the source manifest disagrees, so drift is visible rather than silently repaired. For skills only the first two path segments are replaced, so each entry's skills/<name> tail survives, and a manifest with no skills array is left alone rather than given an empty one.

skills[].folder is in the same pass because it fails more quietly. SeedSkillFolderAsync opens with if (!Directory.Exists(folderAbs)) return;, so a stale value returns without logging. On a fresh host the extension loads and the skill is never seeded. On a host still holding the old version, the old skill is seeded.

Also corrects 02-authoring-guide.md, which told you to bump version on every backend code change without mentioning either path.

No behavior change for a manifest that was already correct, which is all of the samples — they're pinned at 0.1.0 and consistent today. Nothing in the loader or its tests is touched, and checked-in manifests are left alone since the derivation happens at bundle time.