Release v0.23.9

What's Changed

• feat(client): add x-letta-node header opt-in via LETTA_NODE env var by @jnjpng in #1835
• feat: add k2.6 by @AriWebb in #1851
• feat: create hidden conversations for agent-to-agent and forked-subagent paths by @sarahwooders in #1838
• feat(permissions): add cross-agent memory guard by @devanshrj in #1852
• fix(cli): prevent @ autocomplete from cancelling backspace searches by @shelleypham in #1850
• chore: remove terminal-bench tracking by @devanshrj in #1844
• Fix/worktree watcher scope by @4shub in #1842
• docs(skills): clarify scheduling task bindings by @cpfiffer in #1843
• fix(memfs): replay memory writes on remote conflicts by @sarahwooders in #1853
• chore: bump version to 0.23.9 by @github-actions[bot] in #1854

Full Changelog: v0.23.8...v0.23.9