Bug: Jira source {{.Body}} always empty — description field not fetched from API

🤖 Kelos User Agent @gjkim42

Problem

The Jira source implementation never fetches the issue description from the Jira API, so `{{.Body}}` silently evaluates to an empty string in all Jira-based prompt templates — even though both the documentation and the official example use it.

Documentation says it works

integration.md (example prompt):
```yaml
promptTemplate: |
Fix the following Jira issue:

{{.Title}}

{{.Body}}
branch: "jira-{{.ID}}"
```

integration.md template variable table:

Code shows it doesn't

internal/source/jira.go:163 — `description` is absent from the `fields` query parameter:
```go
params.Set("fields", "summary,status,labels,comment,issuetype")
```

internal/source/jira.go:47-53 — `jiraIssueFields` struct has no `Description` field:
```go
type jiraIssueFields struct {
Summary string `json:"summary"`
Status *jiraStatus `json:"status,omitempty"`
Labels []string `json:"labels"`
Comment *jiraComments `json:"comment,omitempty"`
IssueType *jiraIssueType `json:"issuetype,omitempty"`
}
```

internal/source/jira.go:96-104 — `WorkItem.Body` is never set when building the result:
```go
items = append(items, WorkItem{
ID: issue.Key,
Number: number,
Title: issue.Fields.Summary,
URL: fmt.Sprintf(...),
Labels: issue.Fields.Labels,
Comments: comments,
Kind: kind,
// Body is never set — always ""
})
```

Impact

Any Jira-triggered task that includes `{{.Body}}` in its prompt template silently drops the issue description. Agents receive only the title and comments, missing the full context needed to do the work correctly.

Suggested Fix

1. Add `description` to the `fields` request parameter:
   ```go
   params.Set("fields", "summary,status,labels,comment,issuetype,description")
   ```

2. Add a `Description` field to `jiraIssueFields`:
   ```go
   type jiraIssueFields struct {
   Summary string `json:"summary"`
   Description interface{} `json:"description,omitempty"` // string or ADF object
   Status *jiraStatus `json:"status,omitempty"`
   Labels []string `json:"labels"`
   Comment *jiraComments `json:"comment,omitempty"`
   IssueType *jiraIssueType `json:"issuetype,omitempty"`
   }
   ```
   Note: Jira Cloud returns descriptions as Atlassian Document Format (ADF) objects (same as comment bodies) while Jira Data Center/Server returns plain strings — the existing `commentBodyToString` helper already handles this distinction.

3. Populate `WorkItem.Body` in `Discover`:
   ```go
   items = append(items, WorkItem{
   // ...
   Body: commentBodyToString(issue.Fields.Description),
   // ...
   })
   ```

4. Add a test covering the `Body` field for both Jira Cloud (ADF description) and Jira Data Center (plain string description).

/kind bug

Start in internal/source/jira.go, reading jiraIssueFields, the fields query in Discover, and the WorkItem construction. Reuse the existing commentBodyToString handling for both Jira Cloud ADF and Data Center plain descriptions. Add coverage for both description formats and verify that {{.Body}} contains the Jira description.