chore: bump pinned Bun version to 1.4.0

[agjs]

Summary

• Bumps the pinned Bun version from 1.3.14 to 1.4.0 across engines.bun (root + packages/core) and every CI workflow's bun-version.
• Opened as its own PR to see what CI surfaces on Linux under 1.4.0 before deciding on any fix — locally on macOS with Bun 1.4.0, packages/core/tests/script-tool.test.ts's "the output drain is bounded when a backgrounded child holds the pipe open" test fails: a reproduction outside the test harness shows proc.exited (and onExit, and even polling proc.exitCode/proc.killed) all block for the full duration a backgrounded grandchild holds the process's stdout pipe open, instead of resolving promptly — so packages/core/src/lib/fs/process.ts's FLUSH_GRACE_MS bound never gets a chance to apply.
• Not yet clear whether this is macOS-specific or reproduces on Linux too — that's what this PR's CI run will tell us.

Test plan

• Watch CI on this PR to see whether the drain-bound test (or anything else) fails under Bun 1.4.0 on the Linux runners
• If it fails on Linux too: root-cause and fix in a follow-up before merging
• If it's macOS-only: note that in the PR and decide whether it needs a fix at all before merging

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