fix: label esc as back in agent detail pager footer (#130)

[vansh2408]

What

Fixes the keyboard shortcut hint in the workflow navigator's agent detail pager footer: esc was labeled summary, duplicating the enter hint. It now reads enter summary · esc back.

Fixes #130

Why

With the pager open, the footer showed enter summary · esc summary, making both keys look like they do the same thing and hiding the fact that Esc is the universal "go back one level" key across the navigator. Every other view already labels it esc back; this was the one deviation.

How

• src/workflow-ui.ts: changed the footerHint pager-open branch label from "esc summary" to "esc back". Key handling is untouched - keyToAction already maps Esc to back, which closes the pager when it is open.
• tests/workflow-ui-pager.test.ts: added a test that opens the agent detail pager and asserts the footer shows enter summary and esc back, and does not show esc summary, so this regression is now covered.

[QuintinShaw]

Thanks — exactly right, and the rendering test asserting the footer content (not just the keymap) is a nice touch. Welcome aboard. Merging.