ci: prevent duplicate PR check from flagging current PR as duplicate

[Raviguntakala]

Problem

When a new PR is opened, the duplicate PR detection workflow searches for similar PRs.
However, the search results include the newly opened PR itself, causing it to be
incorrectly flagged as a "duplicate" of itself.

Solution

• Added excludePR parameter to the github-pr-search tool to filter out a specific PR
• Updated the workflow to pass the current PR number explicitly via CURRENT_PR_NUMBER
• Updated agent instructions to use this parameter

[github-actions]

The following comment was made by an LLM, it may be inaccurate:

Results

No duplicate PRs found.

The search results only return the PR itself (PR #6924: "fix: prevent duplicate PR check from flagging current PR as duplicate"), which is expected since we're searching for it.

The other results from the first search (PRs #6905, #2483, #4879, #5170, #5779, #6403) are unrelated PRs that matched the broad "duplicate PR detection" keyword search but are not actually about the same issue.

Conclusion: This PR appears to be unique and addressing a specific issue with the duplicate PR detection workflow. There are no existing open PRs that appear to be duplicates or addressing the same problem.