fix(skills): set per-skill directory mode to 0755 through the pinned handle

[XieX]

Same finding as the Bugbot comment on launchdarkly/python-ai-sdk#87 (comment 4232816271). The gap is identical in this SDK.

What was wrong: openOrCreateDirectory in safe-fs.ts created <root>/<key>/ with mkdir(directory, 0o755), but the process umask still masks that mode, and nothing set it afterwards. Under umask 0077 the directory came out 0700, so an agent running as a different identity couldn't read SKILL.md, even though writeSkills reported success. The README's privilege-separation section promises per-skill directories at 0755 no matter what the umask is.

Fix: after a successful create, the code sets the mode to 0755 with FileHandle.chmod (that is, fchmod) on the handle openDirectoryNoFollow already returns, the same way FILE_MODE is applied to files. It never chmods the path, because a swap between the mkdir and the open could redirect a path-based chmod.

• The mode is only changed when this call created the directory. An existing <root>/<key>/ keeps its mode.
• The managed root's mode is still up to the user, as the README says. It's created by a separate mkdir(root) in skills-fs.ts and isn't touched here.
• A setgid bit the new directory inherited is kept. On Linux, a directory created under a setgid parent gets the parent's S_ISGID bit, and fchmod uses exactly the bits passed, so a bare 0755 cleared it. Files written inside would then get the reconcile identity's primary group instead of the shared group. The mode is now (mode & S_ISGID) | 0755, with the current mode read through handle.stat() on the same descriptor. macOS doesn't propagate the bit, so nothing changes there.
• The symlink and not-a-directory refusals are unchanged. If the chmod fails, the handle is closed before the error is rethrown.

Tests (skipped on win32):

• creates per-skill directories as 0755 even under a restrictive umask runs writeSkills under process.umask(0o077) and asserts the directory is 0755 and SKILL.md is 0644. It fails without the fix.
• keeps a setgid bit the new skill directory inherited makes the root 2755 and asserts the new skill directory is 2755. It's skipped unless a probe directory under that root actually inherits the bit, so it runs on Linux and skips on macOS. On Linux (Docker, node:22) it fails without the setgid change. fs.constants has no S_ISGID, so safe-fs.ts defines it locally.
• leaves the mode of an existing directory alone checks that an existing 0700 directory stays 0700.

Full suite: 1394 passed on macOS (the setgid test skips) and 1405 passed on Linux. biome and tsc are clean.

The Python fix is in a separate PR, linked below. The spec lines are in launchdarkly/ai-sdks-monorepo#53 (merged) and its setgid follow-up.

🤖 Generated with Claude Code

[XieX]

Python counterpart: launchdarkly/python-ai-sdk#154 · Spec: https://github.com/launchdarkly/ai-sdks-monorepo/pull/53

[XieX]

Setgid follow-up to the spec: https://github.com/launchdarkly/ai-sdks-monorepo/pull/54 (the first spec PR, #53, is merged).