[CLI] copilot --resume "Name With Spaces" fails silently with exit 1

## Summary
`copilot --resume ""` fails with exit code 1 and no output whenever `` contains spaces, even when the name is an exact, case-correct match for an existing session created via `--name`. Names without spaces (e.g. kebab-case) resolve correctly.

This contradicts the documented usage in `copilot --help`:

```
# Resume a session by name
$ copilot --resume="my feature"
```

That example cannot work today.

## Version
- Copilot CLI **1.0.61**
- Windows 11, PowerShell 7
- Launched via the `agency` wrapper (the standard Windows install)

## Reproduction

Resuming with the command copilot provides after `/exit`

But with just `copilot --resume`:

```powershell
# 1. Create a session with a spaced name
copilot --name "Spaced Name Session" -p "echo init" --allow-all-tools --no-ask-user
# exit 0, session created. workspace.yaml shows: name: Spaced Name Session

# 2. Resume by that exact name
copilot --resume "Spaced Name Session" -p "echo go" --allow-all-tools --no-ask-user
# Observed: exit 1, no stdout, no stderr.
```

Control (changes only one variable — removing the spaces):
```powershell
copilot --name "UnspacedSession" -p "echo init" --allow-all-tools --no-ask-user
copilot --resume "UnspacedSession" -p "echo go" --allow-all-tools --no-ask-user
# Observed: exit 0, session resumes correctly.
```

Both `--resume "Name"` and `--resume="Name"` forms fail identically for the spaced case. Failure also reproduces against a real long-running session whose `name` contains spaces.

## Expected
Either:
1. Spaced names resolve correctly (matching the documented example), OR
2. The CLI emits a clear error explaining the lookup failed and exits non-zero.

Current silent exit with no diagnostic output sends users down the wrong debugging path: shell quoting, terminal lock, session in-use, version skew. Each of those is a plausible suspect for an exit-1-with-no-output failure.

## Workarounds (none acceptable as a fix)
- `copilot --resume <7-char-id-prefix>` works but defeats the purpose of friendly names
- `copilot --resume` (interactive picker) works but requires UI interaction
- Rename to kebab-case works but loses readability

## Why this matters
The documented and expected workflow for users who name sessions with descriptive labels (the example in `--help` is literally `"my feature"`) is broken. Every user who follows the docs hits a wall. The silent failure mode multiplies the cost.

Beginne am Einstiegspunkt `copilot --resume` und reproduziere den dokumentierten Fall mit Leerzeichen im Namen zusammen mit der nicht durch Leerzeichen getrennten Kontrolle in PowerShell. Verfolge die Namenssuche und überprüfe, dass Namen mit Leerzeichen entweder erfolgreich fortgesetzt werden oder eine klare Diagnose statt eines stillen exit 1 erzeugen.

Vom Indexierungsmodell aus dem Issue-Text verfasst.