fix(llms): serve well-formed llms.txt, remove Mothership + dead static files

Both the marketing site and docs site's llms.txt validator errors ("does
not appear to contain any links") traced to the same root cause: a static
public/llms.txt shadowed a better-written, already-existing dynamic
app/llms.txt route, and every "link" in the static files (and in the
docs app's auto-generated route) was bare `label: url` text, not Markdown
link syntax - so a strict Markdown-link parser found zero matches even
though URLs were visibly present.

- apps/sim: delete public/llms.txt (dead code, shadowing the properly
  Markdown-linked app/llms.txt route.ts, confirmed via production headers
  showing the static file was what actually served). Fix llms-full.txt's
  Links/Support/Legal sections to use [label](url) syntax, correct a
  stale "Next.js 15" reference, and replace "Mothership" with "Chat" per
  the constitution's language rules.
- apps/docs: same shadowing issue - delete the orphaned public/llms.txt
  (also still said "Mothership"). Fix the auto-generated per-page link
  list in app/llms.txt/route.ts to emit [title](url) instead of
  "title: url" for every documentation page.

Author: waleedlatif1