Add --context-source flag to bypass broken context_window data.

[motlin]

The context_window field from Claude Code is currently broken (anthropics/claude-code#13783) - it reports cumulative session tokens instead of current context usage, producing impossible readings like 170% after compaction.

Request: Add --context-source flag similar to the existing --cost-source:

• auto (default): prefer context_window, fall back to transcript
• ccusage: always calculate from transcript
• cc: always use Claude Code data

Workaround: jq 'del(.context_window)' before piping to ccusage forces transcript calculation.

Ref: #749 comment by @shailendher