Copy an image to clipboard (screenshot, browser, etc.) and paste into
the Hermes CLI. The image is saved to ~/.hermes/images/, shown as a
badge above the input ([📎 Image #1]), and sent to the model as a
base64-encoded OpenAI vision multimodal content block.

Implementation:
- hermes_cli/clipboard.py: clean module with platform-specific extraction
  - macOS: pngpaste (if installed) → osascript fallback (always available)
  - Linux: xclip (apt install xclip)
- cli.py: BracketedPaste key handler checks clipboard on every paste,
  image bar widget shows attached images, chat() converts to multimodal
  content format, Ctrl+C clears attachments

Inspired by @m0at's fork (https://github.com/m0at/hermes-agent) which
implemented image paste support for local vision models. Reimplemented
cleanly as a separate module with tests.

Fixes NousResearch#633

Problem:
- Sequential numbering gaps (e.g., NousResearch#1, NousResearch#2, NousResearch#5, NousResearch#8) confuse users
- 200 char truncation too aggressive
- Tool messages completely hidden with no indication

Fix:
1. Use separate counter for displayed messages only
2. Skip tool messages but show count at end
3. Skip system messages
4. Increase truncation to 300 chars
5. Display 'N tool messages hidden' summary

Impact:
- Consistent numbering: NousResearch#1, NousResearch#2, NousResearch#3, NousResearch#4
- Users know when tool calls occurred
- More context visible per message

Three bugs fixed in gateway fallback chain:

1. _load_fallback_chain() called build_auto_chain() which scanned env vars
   without primary provider context, causing the active provider to appear
   in its own fallback list and triggering API mode switches mid-conversation
   (anthropic_messages -> chat_completions) on transient errors.
   Fix: return None when no explicit config exists.

2. Fallback notify callback captured asyncio event loop inside run_sync()
   which runs in a thread pool via run_in_executor. Python 3.11+ raises
   RuntimeError('no running event loop') in worker threads.
   Fix: build the callback in the async _run_agent context after
   loop = asyncio.get_event_loop(), before run_in_executor call.

3. When no fallback chain was configured, AIAgent.__init__ auto-built one
   from env vars (same bug as NousResearch#1 but inside the agent).
   Fix: always pass explicit FallbackChain(entries=[], enabled=False)
   when gateway has no chain configured.