[bug] text typed during a turn is lost when the agent completes

TASK.md

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+# Task
+
+Resolve issue #33: [bug] text typed during a turn is lost when the agent completes
+
+## Issue body
+
+### What happened
+While the agent is working, the user types into the message bar (to steer the
+next turn or queue a follow-up). When the agent transitions from working to
+completed, that in-progress text is deleted from the message footer and the bar
+resets to empty. Anything the user was mid-typing is gone.
+
+### Steps to reproduce
+1. Start an interactive sesh session.
+2. Submit a request so the agent is actively working.
+3. While it works, type some text into the message bar.
+4. Wait for the agent to finish (transition to completed).
+
+### Expected
+The text the user is typing should be preserved in the message bar at all
+times, including across the working-to-completed transition. The buffer is the
+user's draft and should survive state changes.
+
+### Actual
+When the turn completes, the input editor is reset and the message bar is
+cleared, discarding the partial message the user was composing.
+
+### Likely location
+The editor buffer is reset in `harness/tui.go`. `beginInput` zeroes `t.buf`
+when the editor re-opens (which fires as the prompt re-shows at completion):
+`t.prompt, t.buf, t.pos, t.mask = prompt, nil, 0, mask`. That clear is
+correct after a submit (`endInput`), but it also destroys a draft that was
+being typed during the turn. The fix likely preserves the current `buf`/`pos`
+when re-opening the editor at completion rather than unconditionally zeroing
+it.
+
+### sesh context
+- build: 33cb3ae
+- provider protocol / model / context window: openai / glm-5.2 / 200000
+- active mods: none
+- tuning overrides: handoff_pct:80 hard_pct:90 max_useful_context:250000 assumed_context:200000 (and a second set: handoff_pct:70 hard_pct:85 max_useful_context:400000) task_depth:3 stuck_after:3 seed_ledger_entries:10 recall_links:50 diff_lines:40 skill_note_off:false update_check:true
+- skills installed: 6
+- tool mods: 1
+- mcp configured: yes
+- invocation: interactive; flags: none
+