chore(deps): move the Bun toolchain to 1.4.0

Every pin: nine workflow files, the setup-bun-playwright composite
default, cli engines and bun-types, lockfile refreshed.

Bun 1.4 aligns node:readline with modern Node and throws
ERR_USE_AFTER_CLOSE from prompt() on a closed interface. The steer REPL
prompted at the tail of each loop turn, and stdin EOF closes the
interface before that runs, so a piped `agentsfleet steer` died on its
own last line — six unit tests caught it, and it was a live crash, not a
test artifact. Guard the prompt rather than the iteration: close fires
while the iterator still has buffered lines, so breaking there drops
them.

Verified on both toolchains: the REPL specs are 17/17 under 1.3.14 and
1.4.0, and the CLI suite is 1624 pass / 0 fail.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>