processor-plugins install --help still says the registry does not serve processors

## Summary

`conduit processor-plugins install --help` still tells users the registry does not serve
processors. It does — `ai.chunk@0.1.0` and `ai.embed@0.1.0` have been in the signed index since
2026-08-02, each with a real artifact and SLSA provenance entry.

`cmd/conduit/root/processorplugins/install.go:133`:

```
Interim offline paths (until the hosted index serves processors):
```

## Why it matters

The `postgres-pgvector-rag` template's prerequisites were just corrected (#2815) to state the
true position: the processors *are* published, and the hosted install is blocked by #2818 rather
than by the registry not carrying them. A user who reads that and then runs `--help` gets the old
story back, one command later.

Two files now disagree about the same fact, and the one that disagrees is the one a user reaches
without knowing the template exists.

## Fix

Rewrite the help text to match reality: the index serves processors; `install` currently fails
with `registry.incompatible_version` because of #2818; the offline/local-build paths remain valid
and are the working route until that lands.

Worth doing **with** #2818 rather than before it — if #2818 lands first, the help text should
simply describe a working hosted install, and this becomes a smaller edit. Filing it separately so
it does not get lost either way.

Found during the review of #2815. Related: #2818.

Open cmd/conduit/root/processorplugins/install.go around line 133 and review the help text against issue #2818. Update the text to state that the index serves processors, explain the current registry.incompatible_version failure, and retain the offline/local-build paths; verify the result with conduit processor-plugins install --help.

Written by the indexing model from the issue text.