Clarify conversation-context counts after session delta filtering

## Problem

Session-aware prompt delivery can filter messages that the live ACP session already received. The rendered heading still uses the post-filter message count with the pre-filter total and truncation state, for example:

```text
[Thread Context (1 of 5 messages)]
```

That does not distinguish messages withheld because they were delivered earlier in the session from messages omitted by the fetch window.

## Desired outcome

Make the conversation-context heading distinguish:

- messages included in this prompt;
- messages withheld because the current session already received them; and
- messages omitted because the fetch was truncated.

Follow-up from #5342 session-context delivery work; intentionally kept out of that bounded PR because the rendering semantics need a product/copy decision.

Start by tracing the conversation-context heading and session delta filtering described here, then review the session-context delivery work in #5342. Determine how the prompt currently represents included, previously delivered, and truncated messages. Done means the rendered heading clearly distinguishes all three categories and reflects the agreed product copy.

Written by the indexing model from the issue text.