[3/11] Add omitempty checker-tag modifier

[cinar]

Problem

Optional fields that must still be valid when present currently require a workaround — building the struct conditionally, or leaning on required-if-style tricks. Every comparable struct-tag validator has a shorthand for "skip validation if this field is empty/zero, otherwise validate normally"; Checker didn't.

Fix

Add omitempty as a checkers-tag pipeline modifier (not a registered checker in the makers map, since it needs the runtime value, not just its own static parameters):

type Profile struct {
	Website string `checkers:"omitempty url"`
}

• reflectCheckFieldWithConfig strips omitempty out of the config string (extractOmitEmpty) and, if the field's own value IsZero(), skips the rest of that field's checks entirely instead of running them.
• It looks at the field's original value, not one already transformed by an earlier normalizer in the same tag — trim omitempty required on " " still fails required after trimming, since a whitespace-only string isn't the zero value to begin with.
• With the @ prefix it applies to a slice/map container instead of its items (@omitempty @min-len:1), following the same convention @ already uses everywhere else in the tag DSL.
• JSONSchema now treats omitempty like a normalizer — skipped rather than recorded in XChecker — since it's a pipeline instruction, not a shape constraint.
• Documented in the README (new "Optional Fields with omitempty" section), including the omitempty+required contradiction: omitempty wins, so required never runs on a zero value — the two shouldn't be paired.

Testing

• New omit_empty_test.go: skip-on-zero, still-checks-on-non-zero, valid-non-zero, the documented omitempty required no-op, the "looks at original value" ordering case, @omitempty on a slice container (empty and non-empty), and CheckWithConfig.
• Extended TestJSONSchemaIgnoresNormalizers to cover omitempty.
• Full suite passes, 100% coverage maintained.
• go vet, gosec, revive clean.

Fixes #200

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Chain note: PR 3 of 11 — stacked on p1-02-error-template-cache (#225), which is itself stacked on the now-merged p1-01-regexp-cache (#224). Merge in order.

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https://claude.ai/code/session_01FLdVmP5daHiknrTW4Geh2i