Create wiki page 'Session File'

Session-File.md

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+# Session File
+
+## Expected format
+
+The downloader expects a Netscape cookie file.
+
+Each cookie row must have 7 tab-separated columns.
+
+Typical source:
+
+- exported browser cookies from a logged-in YouTube session
+
+## Validation rules
+
+Before any download starts, the project checks:
+
+- the file exists
+- the format is valid Netscape cookie format
+- the file contains YouTube or Google domain cookies
+- enough fresh auth cookies are present
+
+Important cookie names include:
+
+- `SID`
+- `HSID`
+- `SSID`
+- `APISID`
+- `SAPISID`
+- `__Secure-1PSID`
+- `__Secure-3PSID`
+- `LOGIN_INFO`
+
+## Validation result
+
+`validate_session_file` returns `SessionValidation`, which describes:
+
+- whether the file exists
+- whether the structure is valid
+- whether the auth cookies look fresh
+- whether the file is usable for download
+- a human-readable message
+
+## Failure modes
+
+Typical failures:
+
+- file path is wrong
+- broken Netscape format
+- no YouTube or Google cookies
+- cookies are expired or incomplete
+
+In those cases the downloader raises `InvalidSessionError` before yt-dlp starts.
+
+## Practical advice
+
+- re-export cookies after the browser session changes
+- keep the file private
+- do not commit cookie files to git
+- prefer a fresh export if YouTube starts returning auth-related errors