Add assign-issue and add-issue-to-sprint commands

[Copilot]

Closes: (issue number to be added)

Adds two new commands for issue management:

CLI commands:

jira assign-issue <issue-key> <assignee>
jira add-issue-to-sprint <issue-key>

MCP tools:

• assign_issue - Assigns issue to specified user
• add_issue_to_sprint - Moves issue to currently active sprint

Implementation:

• assign-issue uses UpdateAssigneeWithContext
• add-issue-to-sprint queries project boards, finds active sprint, calls MoveIssuesToSprintWithContext
• Both commands implemented in CLI (main.go) and MCP server (mcp.go) modes

Tradeoffs:

• Sprint assignment uses first board for project - works for typical single-board setups, may need refinement for multi-board projects
• No board selection parameter to maintain command simplicity

Original prompt

I want new commands:

• assign-issue
• add-issue-to-sprint - add the issue to the current sprint

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