fix(vscode): reuse IPC server port across restarts

[backnotprop]

Summary

• Persist the IPC server port in workspaceState and try to rebind to it on extension activation
• Falls back to a random port if the preferred one is taken (e.g., another VS Code window claimed it)
• Fixes restored terminals having a stale PLANNOTATOR_VSCODE_PORT after VS Code restart

Closes #252

Test plan

• Install extension, open terminal, note echo $PLANNOTATOR_VSCODE_PORT
• Close and reopen VS Code — verify the same port is reused
• Trigger plannotator in a restored terminal — verify it opens in a webview panel
• Open two VS Code windows on different workspaces — verify no port collision

🤖 Generated with Claude Code

[backnotprop]

Code review

No issues found. Checked for bugs and CLAUDE.md compliance.

🤖 Generated with Claude Code

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