feat(mobile): add Discuss button to inbox reports

[Gilbert09]

Summary

• Ports the desktop "Discuss" affordance (feat(code): add Discuss button to inbox reports #2241) to mobile: tapping Discuss on an inbox report opens a sheet where the user can optionally type a first question, then routes to the new-task composer with the agent-fetch-the-report prompt prefilled and the signal_report id attached.
• Moves buildDiscussReportPrompt into @posthog/shared and parameterises it on the deep-link URL so each host passes its own scheme (posthog-code:// on desktop, posthog:// on mobile) without forking the wording. The desktop wrapper keeps its previous isDevBuild signature so all existing callers stay unchanged.
• Fires the same Inbox report action analytics event desktop does with action_type: "discuss", surface: "detail_pane", has_question, and the report id, so the discuss surface shows up alongside the other inbox actions.

Test plan

• Open an inbox report on mobile, tap Discuss: sheet appears with text input
• Submit empty → composer opens with the "brief readout" variant of the prompt and the repo + signalReport params prefilled
• Submit with a question → composer opens with the "answer this first" variant
• Confirm the new task is tagged with origin_product: "signal_report" and signal_report_id on the API side once sent
• On desktop, confirm the existing Discuss button still works (the wrapper now delegates to the shared util)
• pnpm --filter code test buildDiscussReportPrompt still passes

[greptile-apps]

_{Reviews (1): Last reviewed commit: "feat(mobile): add Discuss button to inbo..." | Re-trigger Greptile}

[github-actions]

Clean additive feature: utility logic extracted to shared package with identical behavior, mobile UI follows the existing DismissReportSheet pattern, null guard on report is in place before the new sheet is rendered.