Support optional logseq_dir and recursive .logseqfolder detection

[google-labs-jules]

• Removed hardcoded default logseq_dir in config.
• Added lua/logseq_mode/utils.lua with get_logseq_root function that detects project root via logseq_dir config or .logseqfolder marker (using vim.fs.root).
• Updated init.lua to use utils.get_logseq_root for autocommands and daily_note.
• Updated formatter.lua to use utils.get_logseq_root for condition check.
• Added tests/test_detection.lua for verification logic.

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PR created automatically by Jules for task 5384706600518546979 started by @Conor-McLeod

[google-labs-jules]

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