feat: add nullable schema annotation

[somaz94]

Closes #248.

Adds a nullable meta-keyword that is a shorthand for adding null to a value's type, so you can write # @schema nullable instead of repeating the inferred type in # @schema type: [string, null].

nameOverride: "" # @schema nullable
# => "type": ["string", "null"]

replicaCount: # @schema type: integer; nullable
# => "type": ["integer", "null"]

It is applied after the comment keyword loop, so it merges null into the final type regardless of the order keywords appear in the comment (the keyword map iterates in non-deterministic order). It is idempotent when null is already present, and nullable: false is a no-op.

Validation:

• go test ./... — all packages pass
• gofmt -l pkg/ and go vet ./pkg/ — clean
• New code coverage: processComment and appendNullType both 100% (table tests for nil / scalar / list / already-null / both keyword orderings / false opt-out / invalid bool)
• docs/README.md updated with a ### Nullable section + TOC entry

Note: item-level items/nullable (mentioned in #248 as combining with #247) is intentionally left as a follow-up to keep this PR a single reviewable slice.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (ca39afe) to head (f6a25be).

Additional details and impacted files

@@            Coverage Diff            @@
##              main      #360   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           20        20           
  Lines         2099      2120   +21     
=========================================
+ Hits          2099      2120   +21     

Flag	Coverage Δ
unittests	100.00% <100.00%> (ø)

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[applejag]

Lots of nice changes @somaz94! I'm only seeing this on my phone for now, and want to look at this via a proper computer before I'm willing to review these PRs

I'm still on vacay until end of July, but I'll try get hold of a computer to review this before then so we don't waste your will for contributing