docs(v5): consolidate guides and remove stale research - #299

v5 documentation mixed current usage with completed implementation plans, unmerged prototypes, and old backtest captures. This consolidates it around the implemented scan → install → VEX workflow, with vendoring for committed patched dependencies.

Targets release/v5-prerelease (the branch for #277).

• Reduce the README from 1,548 lines to 136, with focused usage, configuration, migration, and development guides. Condense the unreleased changelog while retaining released entries.
• Remove 19 design/research documents and six historical JSON reports (about 4.5 MB). Keep implemented configuration and Go behavior in maintained guides; preserve package-manager boundaries, fixtures, runners, and the executable vlt tables/coverage map. Historical proposals and results remain in Git history.
• Correct stale hosted-ledger rollback guidance, get defaults, removed environment aliases, installation channels, and claims about vulnerability coverage and offline installs. Update links and test runbooks.

There are no CLI behavior changes. Rust changes only update documentation comments; the Python change removes obsolete snapshot links from generated report prose.

Validation:

• cargo build --locked -p socket-patch-cli
• 116 tests passed across cli_parse_scan, cli_parse_get, cli_parse_main, and cli_config_fallback
• Python script suite: 125 tests, 124 passed and one Linux/glibc-only test skipped on macOS
• 174 local Markdown links and anchors checked; all nine public subcommand help pages rendered
• Configuration example accepted by the built CLI; documented policy keys checked against the parser
• bash scripts/release-lint.sh --sync-only and git diff --check passed

Native installer and production-service matrices were not rerun for this documentation cleanup.