fix(mcp): reject unknown pre-positional flags in add

[afjcjsbx]

📝 Description

Fixes mcp add argument parsing when root-level persistent flags are passed before the subcommand, such as --no-color.

mcp add uses DisableFlagParsing: true, so inherited root flags could leak into the custom parser and be misinterpreted as positional arguments. In practice this broke http/sse server registration and could also silently save incorrect stdio server names.

This PR:

• strips inherited root flags before mcp add parses its own arguments
• returns an explicit error for unknown --prefixed arguments seen before the required positional arguments
• adds regression tests for the reported repro and the unknown-flag case

🗣️ Type of Change

• 🐞 Bug fix (non-breaking change which fixes an issue)
• ✨ New feature (non-breaking change which adds functionality)
• 📖 Documentation update
• ⚡ Code refactoring (no functional changes, no api changes)

🤖 AI Code Generation

• 🤖 Fully AI-generated (100% AI, 0% Human)
• 🛠️ Mostly AI-generated (AI draft, Human verified/modified)
• 👨‍💻 Mostly Human-written (Human lead, AI assisted or none)

🔗 Related Issue

Fixes #3041

📚 Technical Context (Skip for Docs)

• Reference URL: mcp add mis-parses global flags into positionals (DisableFlagParsing), breaking http/sse adds and silently mis-naming stdio servers #3041
• Reasoning: mcp add disables Cobra flag parsing and parses raw args manually. When a root persistent flag like --no-color appears before mcp add, it can arrive unconsumed in parseAddArgs and be incorrectly treated as the server name. This change filters inherited flags before custom parsing and rejects unknown pre-positional flags instead of silently demoting them to positionals.

🧪 Test Environment

• Hardware: Local development machine
• OS: macOS
• Model/Provider:
• Channels: CLI

📸 Evidence (Optional)

Click to view Logs/Screenshots

GOCACHE=/tmp/picoclaw-gocache go test ./cmd/picoclaw/internal/mcp -count=1
ok  	github.com/sipeed/picoclaw/cmd/picoclaw/internal/mcp	1.162s

Regression coverage added for:

• inherited root flag before mcp add positionals (--no-color)
• unknown flag before required positionals returns a clear error

☑️ Checklist

• My code/docs follow the style of this project.
• I have performed a self-review of my own changes.
• I have updated the documentation accordingly.

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