feat: restore issue-tracker console script as migration hint

Callers using the pre-rename `issue-tracker` command now get a clear
stderr message pointing them at `issue-core` (or the short `issue`
alias) and exit code 2, instead of `command not found`.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

pyproject.toml

@@ -78,6 +78,7 @@ Repository = "https://github.com/coulomb/issue-core.git"
 [project.scripts]
 issue = "issue_core.cli.main:main"
 issue-core = "issue_core.cli.main:main"
+issue-tracker = "issue_core.cli.legacy:issue_tracker_hint"
 
 [tool.setuptools.packages.find]
 include = ["issue_core*"]