docs: final CST-agent → Hermit migration notice (0.10.1)

Point readers at WHC/Hermit as canonical; add MIGRATION.md and
migrate_remote_to_hermit.sh so existing checkouts can retarget origin.

Co-authored-by: Cursor <cursoragent@cursor.com>

scripts/bootstrap_linux.sh

@@ -128,7 +128,7 @@ install_from_release() {
 }
 
 main() {
-  local repo_url="${CST_AGENT_REPO:-https://git.whcself.xyz/WHC/CST-agent.git}"
+  local repo_url="${CST_AGENT_REPO:-${HERMIT_REPO:-https://git.whcself.xyz/WHC/Hermit.git}}"
   local prefix="${CST_AGENT_PREFIX:-$HOME/.local}"
   local dir="" pull=1 changed=0 mode="${CST_INSTALL_MODE:-source}" arg
   for arg in "$@"; do