[BUG] utilities/add_*.sh scripts have drifted from the current repo layout

[mattstratton]

The organizer helper scripts in utilities/ have accumulated several bugs as the repo layout changed. None of them are fatal on a good day, but each one produces a silent wrong result or an abort partway through, which is a rough experience for the non-developer organizers these scripts exist to help.

Found while documenting the repo for AI coding assistants. Each is independently fixable.

• add_speakers.sh and add_organizers.sh create the image directory in the wrong tree. Both run mkdir -p ../static/events/$event_slug/{speakers,organizers} but then cp the image into ../assets/events/.... Since both scripts run under set -e, the cp fails and the whole script dies partway through a record if the assets/ directory does not already exist.

• The nav-uncomment substitutions are silent no-ops. add_speakers.sh and add_program.sh look for # - name: speakers / # - name: program (two spaces after the #), but the template in utilities/examples/data/events/main.yml has # - name: speakers (leading space, one space after the #). Nothing matches, nothing is replaced, and string_replace returns 0 either way — so the nav entry stays commented out and the organizer has no idea.

• add_sponsors.sh has a malformed test. Line reads if [ ! -d ../assets/sponsors/$sponsor_initial/]; then — no space before the ]. Bash prints [: missing ']', the test returns non-zero, the mkdir -p is skipped, and the following cp fails if the shard directory does not exist. It works today only because assets/sponsors/{0-9,a-z} all happen to exist already.

• add_program.sh appends blindly. It does cat examples/templates/program.yml >> main.yml, so a second run produces a file with two program: keys.

• add_new_event.sh can leave static/_redirects half-edited. The content-directory duplicate check runs before _redirects is modified, but the data-directory check runs after. A rerun that trips only the second check has already mutated _redirects.

• add_speakers.sh hardcodes .png. Both the "does this image already exist" check and the destination filename assume PNG, so handing it a JPEG writes JPEG bytes to a .png filename. utilities/README.md says headshots may be .png or .jpg, and real events contain both (plus .webp now).