feat: add gitignore option to exclude ignored files from sources/generates

When `gitignore: true` is set at the Taskfile or task level, files
matching .gitignore rules are automatically excluded from sources and
generates glob resolution. This prevents rebuilds triggered by changes
to files that are in .gitignore (build artifacts, generated files, etc.).

Uses go-git to load .gitignore patterns including nested .gitignore
files, .git/info/exclude, and global gitignore configuration.

testdata/gitignore/.gitignore

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+ignored.txt

testdata/gitignore/Taskfile.yml

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+version: '3'
+
+gitignore: true
+
+tasks:
+  build:
+    cmds:
+      - cp ./source.txt ./generated.txt
+    sources:
+      - ./*.txt
+      - exclude: ./generated.txt
+    generates:
+      - ./generated.txt
+    method: checksum
+
+  build-no-gitignore:
+    gitignore: false
+    cmds:
+      - cp ./source.txt ./generated.txt
+    sources:
+      - ./*.txt
+      - exclude: ./generated.txt
+    generates:
+      - ./generated.txt
+    method: checksum

testdata/gitignore/source.txt

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+source content