fix(session-ui): render sequence labels with semicolons

[Hona]

• The reported sequence diagram fell back to code because Mermaid treats ; in Check permissions; create pending request as a statement separator.
• Retry rejected sequence diagrams with message-label semicolons escaped as #59;. Keep valid diagrams, existing entity escapes, the original Markdown, and unrecoverable-input fallback unchanged.
• Use the reported request/reply diagram in the production timeline story.

Before	After

[Copilot]

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