`.lashignore` is undiscoverable: not in --help, config, or the W_INDEX_ORPHAN message

[fohara]

.lashignore already does exactly what I needed, and I could not find it from anywhere lash points you.

What happened

I added a content/ directory of non-task Markdown to a project that uses lash (prose that ships with the site, not tasks). Every file in it now shows up in lash lint:

warning[W_INDEX_ORPHAN]: File '/…/content/writing/a-post.md' is not referenced in the root index
  at /…/content/writing/a-post.md:0:0

One warning per file, growing by one with every post, in a directory that will never contain a task.

Looking for a way to exclude it, I checked lash --help, lash lint --help, and lash config list. None of them mention any ignore or exclude mechanism, so I concluded there wasn't one and wrote a note in our own docs telling people to read past the warnings.

That was wrong. .lashignore is implemented, works, and is exactly right:

$ printf 'content/\n' > .lashignore
$ lash lint     # content warnings gone, 5 warnings -> 4

It's honored in both lash-db/src/walker.rs:399 and lash-cli/src/utils/file_discovery.rs:64, it has a test (test_lashignore_respect), and lash's own repo uses one — lash.index.md:16 mentions it in passing. It just isn't reachable from any surface a user is looking at when they hit the warning.

Suggested fix

Wherever it is cheapest:

• Mention .lashignore in lash lint --help (and ideally in the top-level --help where .gitignore behavior would be described).
• Say it in the W_INDEX_ORPHAN message itself: not referenced in the root index (add to .lashignore if this is not a task file). That is where the user actually is, and it would have saved the whole detour.
• Cover it in the README/user docs; today it only appears in docs/agent-workflows.md, devlog.md, and tasks/.

Second, separable thing

While chasing the above, lash explain — the one place the diagnostic output tells you to go — does not know the codes that lash lint actually emits:

$ lash explain W_INDEX_ORPHAN
Error: Unknown error code 'W_INDEX_ORPHAN'
Run 'lash explain --list' to see all available error codes.

$ lash explain E_LINK_NOT_FOUND
Error: Unknown error code 'E_LINK_NOT_FOUND'

--list shows 46 codes, of which exactly one is a warning (W_SEM_DOC_FRAGMENT). So a user who follows the advice in the error output lands on a dead end at the moment they are already confused. Happy to split this into its own issue if you'd rather track it separately.

Environment

lash 0.4.0 (Homebrew/cargo install, macOS 15.5). Project has a root lash.index.md and task files under tasks/.