config: add hideSessionBar, drop the legacy config fallback

[hbrooks]

What

Two config changes:

hideSessionBar setting. Setting "hideSessionBar": true at the top level of ~/.ellipsis/config.json removes the session nav from the bottom of the interactive UI. It is a config-file setting only, by design: no env var, no CLI flag. When hidden:

• the nav band is not rendered and the chat window gets its rows
• esc / down-arrow at the chat's bottom edge no longer move focus (there is nothing to focus)
• the 5s session-list poll is skipped, since the nav was its only consumer

The setting lives at the top level rather than per host: it describes the terminal in front of the user, which is the same whichever host is active.

Legacy config fallback removed. The pre-0.17 read-only fallback to ~/.config/ellipsis/config.json is gone; the CLI now reads only ~/.ellipsis/config.json (or ELLIPSIS_CONFIG_DIR). We are pre customer, so we move straight to the ideal end state instead of carrying migration paths. Anyone with a config only at the old location will need to agent login again.

Testing

• 4 new unit tests for hideSessionBar() (default false, reads true, non-boolean is false, survives a host write); test/config-legacy.test.ts deleted with the fallback
• typecheck and full suite pass (436 tests)
• verified with an offline Ink render harness: with the flag set, the "+ New session" row and nav hint are absent and the composer still renders; without it, output is unchanged