feat(core): add message queue for user input during inference

[bug-ops]

Summary

• Add bounded FIFO message queue (max 10) to agent loop for user input during model inference
• Extend Channel trait with try_recv() (non-blocking poll) and send_queue_count()
• Merge consecutive messages within 500ms window
• TUI: queue badge indicator [+N queued], Ctrl+K to clear queue
• TelegramChannel: implement try_recv() via mpsc
• Extract process_user_message() from agent run loop

Closes #221

Test plan

• 8 new queue unit tests (enqueue, merge, overflow, drain, clear, FIFO order)
• 3 TUI channel tests (try_recv, send_queue_count)
• All 1249 existing tests pass, 0 regressions
• Clippy clean, fmt clean
• Manual: verify TUI badge appears when typing during inference
• Manual: verify Ctrl+K clears queued messages

[codecov-commenter]

Codecov Report

❌ Patch coverage is 80.76923% with 60 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
crates/zeph-core/src/agent.rs	85.53%	34 Missing ⚠️
crates/zeph-tui/src/app.rs	10.00%	9 Missing ⚠️
crates/zeph-tui/src/widgets/input.rs	0.00%	9 Missing ⚠️
crates/zeph-channels/src/telegram.rs	0.00%	8 Missing ⚠️

@@            Coverage Diff             @@
##             main     #224      +/-   ##
==========================================
+ Coverage   81.32%   81.36%   +0.04%     
==========================================
  Files          78       78              
  Lines       22174    22426     +252     
==========================================
+ Hits        18034    18248     +214     
- Misses       4140     4178      +38     

Files with missing lines	Coverage Δ
crates/zeph-core/src/channel.rs	78.16% <100.00%> (+2.99%)	⬆️
crates/zeph-tui/src/channel.rs	100.00% <100.00%> (ø)
crates/zeph-tui/src/event.rs	67.30% <ø> (ø)
crates/zeph-channels/src/telegram.rs	34.95% <0.00%> (-1.18%)	⬇️
crates/zeph-tui/src/app.rs	79.57% <10.00%> (-0.89%)	⬇️
crates/zeph-tui/src/widgets/input.rs	0.00% <0.00%> (ø)
crates/zeph-core/src/agent.rs	67.79% <85.53%> (+1.51%)	⬆️

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