Support task string shorthand and command string[] shorthand

[crusty-voidzero]

Summary

Implement the two config simplifications discussed in Discord:

1. Allow task shorthand:"some-task": "... command" (equivalent to "some-task": { "command": "... command" })
2. Allow command to accept string[] as a shorthand for command1 && comand2 && ....

Notes

• The task string shorthand should keep the same default caching behavior as the object form (run.cache still overrides, default remains true).
• Docs should make sequencing semantics explicit.
• These two shorthands can be combined: "some-task": ["cmd1", "cmd2"]
• Verifiable by plan snapshot tests. (no need for e2e tests)

[jong-kyung]

@branchseer I'd like to work on this issue, but I don't have permission to self-assign. Could you assign it to me?