agent start: decide the base-tree grant rule for a workspace that is not a worktree #671

Follow-up from #648 (merged d981d48).

agentLaunchOptions refuses any launch grant that overlaps the base working tree for a registered worktree and for an unregistered worktree with a regular .git file. A workspace that is not a git worktree at all (binding kind: "none", e.g. a plain directory or an absent workspace) skips that check, so its launch can keep a cwd grant that is the base repository directory (packages/cli/src/utils/nono.ts ~366; current test at packages/cli/test/agent-launch-options.test.ts ~362).

Decide and test: either a plain or absent workspace must also never receive a grant overlapping a known base repository (and the check applies to it), or document why a non-worktree launch may run in the base directory. Either way, add the test that pins the chosen behaviour.

Refs #648