feat(inbox): clarify agent-session actions and gate them on personal GitHub

[Twixes]

Problem

In the inbox, the "Discuss" / "Ask about this report…" copy doesn't make it clear that the button starts an agent session (people read it as leaving a note for other humans). On top of that, these actions author commits as the user, so they need a personal GitHub integration — but when one is missing the user only finds out via a toast after clicking. The pull request detail was also missing the "copy sharing link" button that the report detail has.

Changes

• Clearer agent copy: "Discuss" → "Discuss with agent", and the question placeholder "Ask about this report…" → "Ask PostHog about this report…".
• Gate on personal GitHub: when the user has no personal GitHub integration, the Discuss and Create PR buttons are disabled with a disabledReason ("…you need to connect GitHub to your account for agent sessions"), and a "Connect personal GitHub for agent sessions" link button appears to the left of Dismiss (linking to GitHub settings). This replaces the need for the old "Connect a GitHub integration to start a discussion" message in the common case.
• Copy link on PRs: added the missing copy-sharing-link button to the pull request detail, matching the report detail.

How did you test this?

• pnpm --filter @posthog/ui typecheck — passes.
• biome check on the changed files — clean.
• pnpm --filter @posthog/ui test (inbox suite) — 697 passing.

Automatic notifications

• Publish to changelog?
• Alert Sales and Marketing teams?

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Created with PostHog Code from a Slack thread

[github-actions]

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[greptile-apps]

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