fix(claude): preserve attachment context when the prompt is a slash command

[herikwebb]

Problem

The CLI query assembly kept only the last user line whenever it started with /, silently discarding the context lines NBI had just appended for the same turn (attachment @-mentions, cell pointers, output context) — the footgun previously documented in extension.py's on_message.

Change

• Extracted the join into assemble_client_query:
  • control-only commands (/clear, /cost, ...) still drop the context — it is meaningless to them;
  • any other command is moved to the front of the query (the CLI only recognizes a command at the start) with the context lines preserved after it, so custom skill/plugin commands receive the user's attachments.
• Promoted claude_sessions' control-command set to a public name (CONTROL_SLASH_COMMANDS, adding /context) and reuse it as the single source of truth; the private alias remains for compatibility.
• 7 unit tests, including a guard that the hidden plan-mode aliases still lead the query.

Testing

Full Python suite passes (1300 tests).

[github-actions]

Automated PR Review

Model: gpt-5.5
Verdict: APPROVE

Changed files:

notebook_intelligence/claude.py
notebook_intelligence/claude_sessions.py
tests/test_claude_client.py

Review result:

No automated findings were found.

Residual risks: the slash-command behavior now depends on a static CONTROL_SLASH_COMMANDS set staying aligned with Claude Code’s built-in/control commands, so future CLI command changes could require updates.

[herikwebb]

Raised upstream as plmbr#388; closing this fork PR. Review history retained here.