Drop string-content truthiness; &&/|| return operands

[myzie]

Summary

• Truthiness (truthy.go): IsTruthy no longer special-cases the string "false". A non-empty string is truthy regardless of content; empty strings stay falsey. bool("false") is now true.
• Logical operators (program.go): && and || return the deciding operand (Python and/or semantics) instead of a coerced bool. Short-circuit behavior is unchanged.

Why

The "false"-string carve-out was a footgun in JSON-heavy contexts and becomes actively harmful once &&/|| return operands (a bare identifier carrying the string "false" would falsey-fall-through unexpectedly).

Operand-returning logical ops compose cleanly with the new truthiness rules:

name || "(none)"       // string fallback
xs   || []             // empty-list default
count || 0             // numeric default

This subsumes a default(x, y) builtin, so per the design doc we are not adding one. Hard bool casts remain available via bool(v).

Docs and tests

• docs/reference/spec.md: Logical and Truthiness sections rewritten.
• llms.txt: bullets updated.
• engine_test.go: added TestEval_LogicalReturnsOperand pinning the new operand-returning behavior.
• script_test.go, boundaries1_test.go, boundaries2_test.go: updated three existing assertions that depended on the dropped string-content rule.
• The templates and higher-order-patterns guides intentionally stay as-is. They will be revisited when the if builtin lands (a later step in the design doc).

Test plan

• go test ./...
• go test -race ./...
• FuzzCompile 20s
• FuzzEval 20s
• FuzzTemplateEval 15s