[11/11] Support validate tag as a fallback for checkers tag

[cinar]

Problem

go-playground/validator has trained the ecosystem to reach for a struct tag named validate. Migrating a codebase already tagged that way to Checker meant renaming every struct tag just to get picked up, even when the tag's contents were already Checker-compatible syntax.

Fix

Add fieldConfig, which reads the checkers tag if present (via Tag.Lookup, so an explicit, empty checkers:"" is honored as "no checks" rather than falling through) and falls back to the validate tag otherwise. Wired into both CheckStruct and JSONSchema's structSchema — the two places that previously read field.Tag.Get(checkerTag) directly — so the fallback behaves consistently for validation and schema generation, not just one of them.

Scope is intentionally narrow: this does not give Checker any understanding of validator's own tag syntax (oneof=a b c, len=5, etc.) — only the tag name is a fallback, not its contents. A struct has to already use Checker-compatible rules under that tag name for this to help. Documented that distinction explicitly in the README so it isn't mistaken for real validator-syntax compatibility.

Testing

• validate_tag_test.go: fallback picks up both checks and normalizers, reports errors correctly, checkers tag takes precedence when both are present, an explicit empty checkers:"" does not fall back, and the fallback works through JSONSchema too.
• Full suite passes, 100% coverage maintained.
• go vet, gosec, revive clean.

Fixes #208

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Chain note: PR 11 of 11 — last one in this chain.

🤖 Generated with Claude Code

https://claude.ai/code/session_01FLdVmP5daHiknrTW4Geh2i

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (fde13bb) to head (8a759de).

Additional details and impacted files

@@            Coverage Diff            @@
##              main      #234   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           64        64           
  Lines         1196      1200    +4     
=========================================
+ Hits          1196      1200    +4     

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