feat!: remove STDIN template; take template as positional arg

[harttle]

Summary

• Remove legacy STDIN template mode from the CLI (planned for v11 / Adds support for options to CLI and improves usability #586 / Remove support for passing template through STDIN, #586 #940).
• Make template a required positional argument; drop --template (breaking, v11).
• Context may still use stdin via --context @-. Unused stdin (including @- as template) is ignored — no special error.
• Update setup docs and README examples.

Fixes #940

Test plan

• npx liquidjs ''{{ hello | capitalize }}'' renders
• npx liquidjs @./file.liquid reads template from file
• echo ''{name:Snake}'' | npx liquidjs ''Hello, {{ name }}!'' --context @- works
• Piped stdin without --context @- is ignored
• npx liquidjs --template ... fails (unknown option)
• Docs no longer use --template or template-via-stdin

[coveralls]

Coverage Report for CI Build 30032482860

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Coverage: 99.528%

Details

• Patch coverage: No coverable lines changed in this PR.

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Coverage Stats

Relevant Lines:	2973
Covered Lines:	2964
Line Coverage:	99.7%
Relevant Branches:	1052
Covered Branches:	1042
Branch Coverage:	99.05%
Branches in Coverage %:	Yes
Coverage Strength:	18997.06 hits per line

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