Remove <channel> support

[AndreiRegiani]

Notes

• pear stage dev . -> no longer supported
• pear touch re-work for pear stage $(pear touch)
• test/13-shutdown.test.js: increased timeout 10_000 -> 15_000 (flaky test solved)
• pear-hyperdb no changes needed, it always used link as primary key, unaware of channel
• Made consistent input validation across all commands: ERR_INVALID_INPUT('A valid pear link must be specified.')

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• Bonus fix

Handled this error when running these commands on a fresh ./pear.dev [main branch]

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$ pear touch

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./pear.dev touch
pear://khessa6n4sq4o7qnay69bc7e5j8wcwrbw4g6anxxskoamq7obday

./pear.dev stage pear://khessa6n4sq4o7qnay69bc7e5j8wcwrbw4g6anxxskoamq7obday examples/terminal

./pear.dev seed pear://khessa6n4sq4o7qnay69bc7e5j8wcwrbw4g6anxxskoamq7obday

./pear.dev run pear://khessa6n4sq4o7qnay69bc7e5j8wcwrbw4g6anxxskoamq7obday

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