Thoughts, step 3: the feed composer can send to "the forge", and the thought shows at the top of the feed as the owner's post #1660

Split from #1060.

Done looks like:

• [ ] The composer's app picker (src/web/studio/feed.ts, tell.ts) has "the forge" as a choice; sending with it stores a thought with source composer through the step-1 action (same-origin checked, escaped on output).
• [ ] The feed shows each thought verbatim as the owner's post, marked "received".
• [ ] One primary action; nothing else on the page moves. The pull's looks show it at phone and desktop widths.
• [ ] A test renders the feed after a composer thought and finds the verbatim text escaped.
• [ ] Start from today's main; run bun x tsc --noEmit and the touched test files before pushing.