v5: align help and migration docs with lockfile-first scans and service-only vendoring #1289

The primary help text still teaches an installed-package/agent-first model, while v5's primary workflow is patching a fresh checkout's lockfiles and then installing.

Verified from the current source and local CLI help:

• Root help and scan -h say: Find patches for installed packages and apply them by rewriting lockfiles to Socket-hosted patched packages. A user with only a lockfile is led to think installation is required first.
• Root help calls repair Agent mode: download missing patch artifacts and clean up unused ones, and groups it only under agent mode. The v5 usage guide also supports repairing missing/corrupt vendored artifacts.
• The migration guide omits an actual v4 compatibility break: --vendor-source build / SOCKET_VENDOR_SOURCE=build is now rejected, and auto no longer falls back to local builds. The v4 argument parser defaulted to auto and offered local builds; the current usage guide documents service-only acquisition.

Sources: Commands help strings, vendored repair documentation.

Before v5, make the first-run instructions match the supported workflow:

• Describe scan as discovering patches from project dependency/lockfiles, including fresh checkouts; installation is only required where the selected mode/ecosystem actually requires it.
• Make it immediately clear that bare scan writes hosted references and scan --dry-run previews them.
• Describe repair as applicable to existing agent or vendored patch artifacts, while keeping the existing limitation that it cannot recreate a lost vendor ledger.
• Keep root, short, and long help consistent with the README/migration guide. No command renaming or new behavior is required.
• Add service-only vendoring to the v5 migration guide: removed build value/environment configuration, auto alias semantics, no local fallback for missing/pending artifacts, and the distinction between reusing healthy committed artifacts offline and fetching new ones.

Filed during the maintainer-requested v5 interface review. This is a small help correction for the normal lockfile workflow, independent of crash recovery or repeated invocation edge cases.