Deliver the instance shell experience #556

Goal

Let users open an interactive shell or run a command in a supported instance from the CLI or Cloud Portal.

Part of datum-cloud/enhancements#900.

Completed work

• Design and API: docs: Propose instance shell sessions #384, feat: Let users open shell sessions into instances #385
• Session handling: feat: Run instance shell sessions from the control plane #387, feat: Serve instance shell sessions from compute cells #390
• CLI and browser clients: feat: Add datumctl compute exec and a browser console client #388, feat: Open instance shells from the Cloud Portal #389
• Reliability and usability: fix: Keep shell sessions off cells clients cannot reach #396, Instance shell: clear endings, command choice and mobile support #398, Add observability for Instance shell sessions #400, feat: Open shell sessions to every project by default #406
• Follow-up hardening: Add per-session subreaper for instance shell cleanup #496, fix: Record each shell session start and end once, even under concurrent writes #522, Stop one instance from flooding the cell API during shell cleanup #523, fix: Stop shell exit watch once its session ends #542, Stop compute exec from turning on compute for a project #543

Remaining bugs and improvements are tracked as separate issues.