Filter internal ChatPeek output from exports

[yfrbl]

This PR tightens ChatPeek's Markdown export so shared conversations only include user-visible content by default.

Changes:

• hide reasoning traces (thoughts, reasoning_recap) unless explicitly enabled
• hide model editable context unless explicitly enabled
• hide tool output unless explicitly enabled
• drop internal tool invocation payloads from Assistant code blocks, including open/search_query-style JSON
• omit redacted tool placeholder messages such as The output of this plugin was redacted.
• add CLI flags to opt back in when needed
• update tests and README to document the new behavior

Verified with the full test suite (124 tests, OK).