feat(cli): /history shows recent user prompts in this session

[emal-avala]

Summary

Adds `/history` (alias `/hist`) — a numbered list of what you've typed in the current session, chronologically oldest→newest so the latest prompt sits near the cursor.

• `/history` — last 10
• `/history 25` — last 25
• `/history all` (or `*`) — everything

Filters out tool_result blocks, compact-summary boundaries, and assistant responses, so the list only contains actual user-authored prompts. Each entry is clipped to ~100 chars on a single line (whitespace collapsed, ellipsis appended for longer ones).

Why

Useful for:

• Jogging your memory about what you've tried when a session gets long
• Finding the exact turn where something went sideways before using `/rewind`
• Skimming recent context before running `/compact`

Test plan

• `cargo fmt --all`
• `cargo clippy -p agent-code --all-targets -- -D warnings`
• `cargo test -p agent-code --bin agent` (103 passed)
• New tests:
  • `preview_user_prompt_collapses_whitespace`
  • `preview_user_prompt_truncates_to_max_chars`
  • `preview_user_prompt_returns_placeholder_for_empty`
  • `parse_history_limit_defaults_to_ten`
  • `parse_history_limit_parses_positive_number`
  • `parse_history_limit_accepts_all`
  • `parse_history_limit_min_one`
  • `collect_user_prompts_skips_tool_results_and_compaction`
  • `collect_user_prompts_skips_whitespace_only`

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