`glob` tool false-negatives on any multi-segment pattern unless prefixed with `**/`

## Describe the bug

The built-in `glob` tool returns "No files matched the pattern." for **any
pattern containing a path separator**, unless the pattern is prefixed with
`**/`. Literal relative paths (`2026/07/26.md`) and wildcarded multi-segment
patterns (`2026/07/*.md`) both fail even though the files exist directly under
the given `paths` root. Single-segment patterns (`inbox.md`, `*.md`) and
`**/`-prefixed patterns work.

This is worse than an inconvenience: the agent trusts the empty result. In one
session the agent checked for a file with its exact relative path, got no
match, and confidently declared the file missing — then acted on that false
premise.

## Affected version

1.0.70-0 (running as the VS Code Agent Sessions CLI backend; macOS arm64)

## Steps to reproduce the behavior

Against a repo containing `2026/07/26.md` and `inbox.md` at the root:

1. `glob` with `pattern: "2026/07/26.md"`, `paths: [""]` →
`No files matched the pattern.` (file exists)
2. `glob` with `pattern: "2026/07/*.md"`, same paths →
`No files matched the pattern.` (31 files exist there)
3. `glob` with `pattern: "**/07/26.md"`, same paths →
`/2026/07/26.md` ✔
4. `glob` with `pattern: "inbox.md"`, same paths →
`/inbox.md` ✔

So the failure is specifically: multi-segment pattern **not** prefixed with
`**/`. It looks like the matcher compares patterns against basenames (or a
single path segment) unless a globstar forces full-path matching.

## Expected behavior

A relative pattern is resolved against the given `paths` roots, per standard
glob semantics: `2026/07/26.md` matches `/2026/07/26.md`. At minimum,
the tool description should say patterns must be `**/`-prefixed to traverse
directories — but fixing the matching is much better than documenting the
trap, since empty results are treated as evidence of absence.

## Additional context

- OS: macOS 26.5.2, arm64
- Observed both from VS Code Agent Sessions and reproduced deterministically
(4 calls above, same session, seconds apart)

Beginne damit, das integrierte glob-Tool und seine Implementierung für Pfadübereinstimmungen zu lokalisieren, und reproduziere dann die vier Muster aus dem Issue anhand eines Repositorys, das die aufgeführten Dateien enthält. Verfolge, wie relative Muster mit den Pfad-Roots verglichen werden. Erledigt ist die Aufgabe, wenn mehrteilige relative Muster vorhandene Dateien finden, während die bestehenden Fälle mit einem einzelnen Segment und **/ weiterhin funktionieren.

Vom Indexierungsmodell aus dem Issue-Text verfasst.