Add read-only mode

[tagplus5]

• Add an opt-in read_only configuration setting and a jiratui ui --read-only session flag.
• Block mutating Jira requests at the shared async and sync HTTP client boundary.
• Continue allowing GET requests and Jira POST endpoints used for search, approximate counts, and expression evaluation.
• Read-only mode is disabled by default and does not replace Jira permissions or a read-scoped API token.