Show annotation counts and next/previous navigation in the file browser

### Feature summary

Provide a quick overview of file annotations and easily navigate to them.

### What problem are you trying to solve?

When a session produces inline annotations (review findings, agent comments) across a diff, there's no way to tell at a glance how many exist or where they are. Today you have to open each file and scroll to find them. On a change touching a dozen files with a handful of annotations, this means:

- **No visible count** — you can't tell whether a file has 0 or 8 annotations without opening it.
- **No total** — you don't know how much review feedback is outstanding overall.
- **No jump** — there's no "go to next annotation" affordance, so triaging means manual scrolling per file.

The result is that annotations get missed, and reviewing them is slower than it should be.

### Proposed solution

1. **Per-file badge in the file browser** — each file row shows a count of its annotations, styled by the highest severity present (blocking / warning / info). Files with zero annotations show no badge.
2. **Aggregate count in the header** — a total like `12 annotations across 5 files`, with unresolved vs. resolved broken out if resolution state exists.
3. **Next/previous navigation** — prev/next controls (plus keyboard shortcuts, e.g. `n` / `Shift+N`) that walk annotations in file-then-line order, crossing file boundaries automatically: opens the target file, scrolls the annotation into view, and focuses it.
4. **Filtering** — optionally collapse the browser to only files that have annotations, and/or filter by severity.

### Workflow impact

_No response_

### Installation context

_No response_

### Additional context

_No response_

Non vengono indicati file, test o punti di ingresso. Inizia individuando il browser dei file e il flusso di dati delle annotazioni esistente, quindi determina come si integrano conteggi, gravità, risoluzione, navigazione e filtraggio. Il lavoro è completo quando sono visibili i conteggi per file e i conteggi aggregati e gli utenti possono spostarsi tra le annotazioni nei vari file secondo il comportamento di navigazione specificato.

Scritto dal modello di indicizzazione a partire dal testo della issue.