feat: cap markdown render width on wide terminals + add --width flag

[howardpen9]

Summary

Fixes #101 — wide terminals make summaries tough to read because lines stretch across the full terminal width.

• Cap default render width at 120 columns: markdownRenderWidth() now applies Math.min(w - 1, 120), so text stays readable even on very wide terminals. This is the default behavior with no flags needed.
• Add --width <columns> CLI flag: Users can explicitly override the render width if they prefer a different value (e.g., --width 80 for narrow output, or --width 200 to use the full width).
• The --width flag works by injecting the value into env.COLUMNS, which flows through the existing terminalWidth() → markdownRenderWidth() pipeline — all 7 call sites automatically get the override with zero changes needed.

Implementation

File	Change
src/run/terminal.ts	markdownRenderWidth() caps at DEFAULT_MAX_RENDER_WIDTH (120)
src/run/help.ts	New --width <columns> CLI option
src/run/runner.ts	Parse --width, validate ≥ 20, inject into env.COLUMNS
tests/terminal.test.ts	3 new test cases for wide terminal capping

Test plan

• vitest run tests/terminal.test.ts — all 7 tests pass (3 new)
• tsc -p tsconfig.build.json --noEmit — clean
• Manual: summarize "https://example.com" on a wide terminal → verify lines cap at ~120 chars
• Manual: summarize "https://example.com" --width 80 → verify narrower output
• Manual: summarize "https://example.com" --width 200 → verify wider output (up to 200)

🤖 Generated with Claude Code

[steipete]

Landed via temp rebase onto main.

• Gate: pnpm -s build && pnpm -s lint && pnpm -s test
• Land commit: fb4e334
• Merge commit: 000b8f9

Thanks @howardpen9!