[bug] text typed during a turn is lost when the agent completes #33

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