Add resource limits to skill packager

[JAORMX]

Summary

The skill packager's collectSkillFiles() function walks a skill directory and reads all file contents into memory before building the OCI artifact. Without limits, a maliciously crafted or excessively large skill directory could exhaust memory during the Build operation exposed via the thv serve API.

The gap

toolhive-core already has comprehensive size limits on the extraction and registry paths:

Limit	Value	Location	Protects
MaxDecompressedSize	100 MB	gzip.go	Decompression bombs
MaxTarFileSize	100 MB/file	tar.go	Oversized archive entries
MaxBlobSize	100 MB	registry.go	Registry push/pull
MaxManifestSize	1 MB	registry.go	Registry push/pull
maxFrontmatterSize	64 KB	packager.go	YAML parsing attacks

And toolhive adds extraction-side limits:

Limit	Value	Location
MaxTotalExtractSize	500 MB	installer.go
MaxFileExtractSize	100 MB/file	installer.go
MaxExtractFileCount	1,000 files	installer.go

However, the packaging (build) side had no file count or aggregate size limit. This meant the memory exhaustion could happen before any of the downstream limits had a chance to trigger.

What this PR adds

Two limits to collectSkillFiles():

• maxSkillFiles = 1,000 — caps the number of files collected. Aligned with MaxExtractFileCount in toolhive so that anything that can be packaged can also be extracted.
• maxSkillTotalSize = 100 MB — caps the total aggregate size of all file contents read into memory. Aligned with MaxBlobSize and MaxDecompressedSize in this package.

Both limits are checked incrementally during the directory walk, so the function fails fast without reading the entire directory.

Test plan

• TestCollectSkillFiles_ExceedsMaxFiles — creates 1,001 files (one over the limit), verifies the error
• task test — all tests pass (including race detector)
• task lint — 0 issues

🤖 Generated with Claude Code