chore(scanner): add gitsecrets ignore file

Initializes .gitsecrets-ignore file to track false positives from secret scanning. Includes documentation header explaining the file format and usage.

.gitsecrets-ignore

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+# GitSecrets Ignore File
+# This file tracks false positives identified by AI evaluation or manually marked.
+# Each line is a JSON object with the following fields:
+# - contentHash: SHA256 hash prefix of the secret content
+# - patternId: The pattern that detected this secret
+# - filePath: Relative path where the secret was found
+# - reason: Why this was marked as a false positive
+# - confidence: AI confidence level (if from AI evaluation)
+# - addedAt: Timestamp when this entry was added
+#
+# You can safely commit this file to share false positive markers with your team.
+# To remove an entry, simply delete the corresponding line.
+
+{"contentHash":"5af30500c6463ec4","patternId":"password-assignment","filePath":"..\\gitcaddy\\internal\\app\\cmd\\register.go","reason":"Manually marked as false positive","addedAt":1769249840525}