fix(mcp): redact and scope native history fallback

[NaNIsaac]

When nothing is captured, terminal_history fell back to native shell history unredacted and ignored here/failed.

• Fallback command text goes through redact(), the same masking append_entry() applies.
• here=true (default): fallback refused with a note, since native history has no cwd.
• failed=true: refused with a note, since native history has no exit codes.
• Tool description and README document the limits.

Fixes #2