fix: duplicate output tag in system prompts, typos in comments and logs

[fix2015]

Found a few issues in the prompts and code:

• both system_prompt.md files (core and extension) had a duplicate `` closing tag at the end, creating ambiguous XML structure that could confuse the LLM when parsing prompt boundaries
• PageAgentCore.ts comment said "will threw" instead of "will throw"
• the GPT-5.5 patch debug log said "GPT-5.4" instead of "GPT-5.5"
• the done action documentation said "one of three cases" but four cases are actually listed below it

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