Add default-operator option (#59)

* Add default-operator option

Add a --default-operator flag controlling how adjacent query terms with no
explicit AND/OR between them are combined. The default remains "and" (a file
must contain every term); "or" matches any term, which is useful for tools
and agents that issue broad multi-keyword queries where the default AND is
often too restrictive and returns zero results.

The operator flows through DoSearch to the query parser, so all modes
(console, TUI, HTTP, MCP) honour it.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

* Filters apply to the whole query under --default-operator=or

With a default-or operator, a query like "foo bar path:pkg" parses as
"foo OR bar OR path:pkg", which unexpectedly OR's the path filter with the
keywords instead of constraining the results. Users expect metadata filters
(path:, ext:, file:, lang:, complexity:, ...) to apply to the whole query.

Tag OR nodes created from adjacent terms as implicit, then hoist filters out
of implicit-OR groupings and AND them at the top, so "foo bar path:pkg"
becomes "(foo OR bar) AND path:pkg". Explicit "foo OR path:pkg" and negated
filters are left untouched.

Hoisting also runs when a parenthesised group closes, which confines a
filter to its group: "(foo path:pkg) bar" becomes "(foo AND path:pkg) OR bar"
while a top-level filter still applies across the whole query.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

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Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>