[Feature]: Support indexing Pull Requests in CLI index command

[Kavirubc]

Problem Statement

The current CLI index command explicitly skips Pull Requests:

if issue.IsPullRequest() {
    continue // Skip PRs
}

This makes it impossible to use the CLI to backfill or test on historical PRs, which is critical for ensuring the bot has context on past code changes and discussions.

Proposed Solution

Update cmd/simili/commands/index.go to:

1. Remove the hardcoded check that skips PRs.
2. Ensure the processIssue function handles PR objects correctly (likely just treating them as issues since they share the same structure for Title/Body).
3. Optionally add a flag --include-prs (defaulting to true or false) to control this behavior.

Alternatives Considered

• Using a separate command index-prs, but this duplicates logic.

Feature Scope

• CLI