feat: post Linear issue link to Slack thread on MCP issue creation (CYPACK-1048)

[cyrusagent]

Assignee: @PaytonWebber (payton)

Summary

When Cyrus creates a Linear issue via mcp__linear__save_issue during a Slack session, a notification message with the issue link is now automatically posted to the Slack thread.

Changes:

• Added a ToolResultCallback mechanism to AgentSessionManager that fires when any tool result is processed, allowing consumers to react to specific tool completions
• Added optional postThreadMessage method to ChatPlatformAdapter interface for mid-session thread notifications
• ChatSessionHandler registers a tool result listener that detects mcp__linear__save_issue results and posts the issue link (with identifier) to the thread
• SlackChatAdapter implements postThreadMessage using the existing SlackMessageService

Design decisions:

• The callback mechanism is generic (not Slack-specific), so other platforms can use it too
• URL and identifier extraction handles both JSON responses and plain text via fallback regex
• Error results are silently skipped (no notification on failed issue creation)
• Callbacks are fire-and-forget with error logging to avoid blocking the main message flow

Resolves CYPACK-1048

Test plan

• Unit tests for extractLinearIssueUrl (JSON, plain text, empty, no URL)
• Unit tests for extractLinearIssueIdentifier (JSON, plain text, no identifier)
• Unit tests for handleToolResultNotification (happy path, wrong tool, error result, missing session, missing adapter method, no URL in result)
• Unit tests for AgentSessionManager.onToolResult callback mechanism (fires on tool result, multiple callbacks, error results marked, callback errors don't break processing)
• All 560 package tests pass
• TypeScript type checking passes
• Manual validation via F1 test drive with Slack session creating a Linear issue

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