feat(extensions): discover installed extensions

[scottlovegrove]

Slice 2 of 7, on top of slice 1.

Discovery walks the extensions directory and works out what each entry is: a symlink or path file is a local install, a .git directory is a clone, anything else is a release binary. It runs on every invocation of the CLI, including the common case of no extensions at all, so it costs one readdir plus a couple of small file reads and never spawns a process or touches the network. The git remote is read out of .git/config rather than by asking git.

Also adds the test fixture helpers, which build real extension directories in a temporary location so later slices can install and run something genuine rather than a mock.

Stack: #521 core · #522 discovery · #523 dispatch · #524 tools · #525 install · #526 upgrade · #527 manager

[doistbot]

This slice adds extension discovery that classifies each entry in the extensions directory (local install via symlink/path file, git clone via .git, or release binary) with a lightweight, read-only scan that runs on every CLI invocation, plus test fixture helpers for building real extension directories. The implementation reuses the existing slice-1 helpers well and covers the main cases. Few things worth tightening:

• Resolve relative targets in path files against the path file's own directory (resolve(dirname(entryPath), target)) instead of process.cwd(), so local installs behave deterministically regardless of where td is invoked.
• Check that .git is a directory (e.g. via the existing isDirectory() helper), not just that it exists, so binary installs containing a .git file aren't misclassified as clones.
• Only treat a missing extensions directory (ENOENT) as "no extensions" and rethrow other readdir failures, so permission/I/O errors don't silently masquerade as an empty install.
• Bound the concurrency of per-entry discovery instead of fanning out with Promise.all, and in the Node fixture prefer process.exitCode (or a sync write) over process.exit so the JSON report isn't truncated on pipes.

I also included a few optional follow-up notes in the details below.

Optional follow-up notes (3)

• src/lib/extensions/discover.test.ts:101: This test unconditionally creates a directory symlink, which fails on Windows without elevated rights or Developer Mode — the exact limitation the Windows-style path-file test works around. Guard it with it.skipIf(process.platform === 'win32') (or another POSIX-only guard) so the suite stays runnable on Windows.
• src/test-support/extension-fixture.ts:73: This fixture re-derives the extension naming conventions that are already the source of truth in src/lib/extensions/source.ts: ${binName}-${name} (line 73), ${binName}-extension.json (line 84), and .${binName}-manifest.json (line 90). Import and use toDirName, authoredManifestFileName, and manifestFileName instead, so the fixture cannot drift from the real on-disk contract.
• src/lib/extensions/discover.ts:113: For git installs, readGitRemote runs after the Promise.all that already reads the authored manifest and state, so the .git/config read is serialized behind those independent reads. Add it as a fourth entry in that Promise.all (resolving only for kind === 'git') so all per-extension file reads complete concurrently.

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[doist-release-bot]

🎉 This PR is included in version 5.4.0-next.1 🎉

The release is available on:

• npm package (@next dist-tag)
• GitHub release

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