build log --follow, streamed to the build page #250

No surface can follow a running build: build log prints what is stored and exits, and the build page renders the log once.

Add build log <owner/name> <n> --follow as the capability, and have the web page stream it without JavaScript.

• Log fan-out: the server receives the runner's log over SSH and stores it; followers need to be woken on each appended chunk rather than re-reading the stored log.
• --follow: print what is stored, then new output until the build reaches an outcome, then the outcome.
• Web: a chunked response that writes the page through <pre class="code buildlog">, flushes each chunk, and closes with the outcome and the footer. compress.go's gzip writer must flush per chunk.
• An explicit end line ("build finished: success"), since the status chip rendered at the top cannot change; a dropped stream shows a reconnect link instead, so it never reads as finished.
• Server write timeouts that tolerate long silent steps; a cap on concurrent followers per account.

Chosen over a Refresh header: live output, no reloads interrupting selection or screen readers, each byte sent once instead of the whole log per reload per viewer, and it lands as a control command every surface shares.