D60 — Client-side bootstrap hook: auto-sync at session start (WP4)

Context. docs/sync.md §Layer 1 described, since before D57, a SessionStart hook that invokes cartographer sync/status at session start — never implemented: D57/D58/D59 built the registration mechanism for KB hooks, but this one never comes from a KB. An artifact generated entirely by the client was needed.

Decision. internal/provisioning/bootstrap.go introduces EnsureBootstrapHook: it materializes a deterministic script (bootstrap.sh, calls cartographer sync --auto-trust) and reuses verbatim registerHookSettings/registerHookConfigTOML/registerOpenCodePlugin (D57/D58/D59) for native registration — zero new code. Reserved name cartographer-bootstrap. Called by doConnect/cmdSync before the manifest fetch, independent of server reachability. "Orphan" protection: the server manifest will never contain this artifact — ComputeDiff explicitly excludes Kind=="hook" && Name==BootstrapHookName from the Removed computation, so the bootstrap is not deleted and recreated at every sync (same principle already used for kind: instructions). Name collision with a same-named KB hook → ignored with a warning, never overwritten. kiro (no native hook mechanism) remains a no-op, degraded to Layer 2. Rationale. Protecting the bootstrap from the diff (instead of having it reappear at every sync) avoids noise (files/config rewritten every round) and a window in which the hook does not exist between prune and rematerialization. Details: docs/sync.md §Layer 1.