fix: load slack project options on demand past the 100-option limit

[Jamyn]

Slack caps static_select at 100 options, so a deployment with more than 100 enabled projects could not open Update Incident, Report Incident, Escalate Case or Report Issue at all. Closes #86.

• project_select switches to an external_select served by a new options-load handler past 100 projects; at or below 100 the payload is unchanged. No project is capped or hidden.
• Report Incident resolved the submitted project from the option's text, which silently filed against the default project whenever display_name differed from name; it now resolves by id.
• Operators must set the Slack app's Options Load URL to /api/v1/<organization>/events/slack/menu (Interactivity & Shortcuts → Select Menus). It is a separate field from the Request URL; without it the project menu opens empty. Documented in configuring-slack.mdx.