chore: revert project-template submodule back to main

## Context

The project-template submodule currently points to the `kbg/temp/miden-day` branch instead of `main`. This branch exists solely to restore symlinks (`AGENTS.md -> CLAUDE.md` and `.agents/skills -> ../.claude/skills`) that were removed in 0xMiden/project-template#22 because the compiler does not support symlinks yet (0xMiden/compiler#1002).

## Proposal

After the compiler issue is resolved, revert the submodule back to project-template `main`:
- Add symlinks back to project-template `main` directly
- Point the agentic-template submodule back to `main`
- Delete the `kbg/temp/miden-day` branches on both repos

Check the current submodule reference in .gitmodules and the git history for the kbg/temp/miden-day branch. Verify that the compiler issue (0xMiden/compiler#1002) is resolved. Update the submodule to point to main, ensure symlinks AGENTS.md -> CLAUDE.md and .agents/skills -> ../.claude/skills are restored, and delete the temporary branches.

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