fix: skip symlink cycles during pack instead of failing with ELOOP

[gagan-53]

Fixes #292.

Problem

When an extension directory contains a symlink cycle (e.g. ln -s . selfloop), mcpb pack aborts with a raw OS error and produces no archive:

ERROR: Archive error: ELOOP: too many symbolic links encountered, stat '.../demo-ext/selfloop/selfloop/.../selfloop'

getAllFiles and getAllFilesWithCount in src/node/files.ts walk the tree with statSync (which follows symlinks) and recurse into anything reported as a directory, with no tracking of already-visited paths, so a cyclic link descends until the OS raises ELOOP.

Fix

Track the real paths (realpathSync) of the directories on the current traversal path. Before recursing into a directory, resolve its real path; if it is already on the traversal path, emit a warning naming the offending path and skip it:

Warning: Symlink cycle detected, skipping: selfloop

Packing then continues and succeeds. Behavior for everything else is unchanged:

• Non-cyclic symlinks (files and directories) are still followed and their targets packed, as before — important for layouts that rely on symlinked directories.
• Only the traversal path is tracked (entries are removed after recursion returns), so two independent links to the same directory are still both packed, as before.
• The new visitedRealPaths parameter is optional and trailing on both exported functions, so the public API is backward compatible.

Testing

• New test/symlink-cycle.test.ts: self-referential symlink, mutually recursive symlinks (A→B, B→A), a non-cyclic symlink still being followed, and coverage of both getAllFiles and getAllFilesWithCount. The cycle tests fail with ELOOP without the fix. Symlinks are created with type junction (works unprivileged on Windows; ignored elsewhere) and the tests no-op if the environment cannot create symlinks.
• yarn test: 129 passed, 9 suites.
• yarn lint: clean.
• Manual repro from the issue: before — ERROR: Archive error: ELOOP: ..., exit 1, no archive; after — warning + successful pack, archive written, exit 0.

[devYRPauli]

I offered on #292 to test this against both cases in the report, so here is the result. Both are fixed, and the non-cyclic control is byte-identical to main.

Setup: main at 70fe3b3, this branch at 4e925f2, macOS, node v24.18.0, yarn build then node dist/cli/cli.js pack.

case	layout	main	this branch
self-referential link	ln -s . selfloop	ELOOP, exit 1, no archive	exit 0, 2 files, warns selfloop
links at each other's parents	a/linkb -> ../b, b/linka -> ../a	ELOOP, exit 1, no archive	exit 0, 2 files, warns a/linkb/linka and b/linka/linkb
control: non-cyclic linked dir	link -> real holding data.txt	exit 0, 4 files, 590 bytes	exit 0, 4 files, 590 bytes

The control archive holds the same four entries on both refs: index.js, link/data.txt, manifest.json, real/data.txt. A symlinked directory is still followed and its target still packed.

The new tests have teeth. I replaced visitedRealPaths.has(realPath) with false to disable detection while leaving the rest of the walk in place. 3 of the 4 tests in test/symlink-cycle.test.ts then fail, and the non-cyclic one still passes, which is the correct split. The full suite is 129 passed across 9 suites on this branch.

Two small notes, neither a blocker.

The warning prints once per walk, so a single cycle is reported twice. getAllFiles and getAllFilesWithCount both walk the tree, so case 1 emits two identical lines and case 2 emits four. Deduplicating is optional, but a user with several cycles will see a long list.

A dangling symlink still fails the pack. ln -s ./nowhere dangling gives ERROR: Archive error: ENOENT: no such file or directory, stat '.../dangling' on both refs, because statSync follows the link and throws before realpathSync runs. That is the same on main, so this PR does not change it. A broken link is the next gap in the same walk, if you want to close it.