Polish diff/CLI output rough edges (deepdiff rendering, absorbed-change hints, arg-diff labeling)

[iamfaham]

Context. Several small diff/CLI output rough edges accumulated across iterations, none blocking but worth cleaning up together:

• deepdiff per-path rendering could be friendlier (currently close to raw deepdiff output).
• The PASSED-line summary has no "model_tools: changed (absorbed)" hint when a model-tool-request change was absorbed into an otherwise-passing structural diff.
• Cross-tool argument diffs don't label which tool call an arg-diff key belongs to when tool names repeat (e.g. model_tool:old->new[i]).
• agentsnap status/CLI wording around "structural change absorbed" could be more consistent between commands.

Proposed. Pass over core/diff.py output formatting and CLI summary lines to address the four points above.

Source: internal backlog, deferred across iterations 6-9.