chore: remove the tracked root-level core symlink to an absolute container path (#7160)

`core` was a tracked symlink (mode 120000) at the repository root whose target
is `/tmp/workspace/objectstack-ai/objectstack/packages/core` -- an absolute path
inside one particular agent container. It dangles on every checkout that is not
literally at that path, including every CI runner.

It was added on 2026-05-25 by `copilot-swe-agent[bot]` inside a commit about
something else entirely ("fix: update spec package.json exports to nested
TypeScript conditions"), so it reads as working-directory residue swept up by a
`git add -A` rather than a deliberate alias. It was the repo's only tracked
symlink.

Nothing resolves through it: no tsconfig `paths`/`references`, no package.json
path field, no workspace glob, no workflow, no Dockerfile, no bundler or vitest
alias, and no import specifier names the repo-root `core`. Because it has been
dangling on every CI runner since it landed, a dependency on it would already
have shown as a red main.

Fixes #7105


Claude-Session: https://claude.ai/code/session_01F8q5J1MQyocgtNspb15fSn

Co-authored-by: Claude <noreply@anthropic.com>

Author: os-project-manager