fix: clamp terminal dimensions to positive integers in attach plan

[adi-IL]

When planAttach is called with missing, zero, or negative pane dimensions (such as during headless runs or uninitialized window metrics), rec.terminalCols === pane.cols and the resize calculation can produce 0, negative, or NaN geometry entries.

This change:

• Clamps cols and rows in planAttach to positive integers with fallback to standard default geometry (80x24) when pane dimensions are non-positive or undefined.
• Clamps dims() in src/attach.js to ensure positive dimensions when reading process.stdout.columns and process.stdout.rows.
• Adds unit tests in test/attach-plan.test.js covering missing, zero, negative, and single-row geometry.

Tests

node --test test/*.test.js
# 286 tests, 0 failures

bash test/cli.test.sh
# 299 tests, 0 failures