Rename `cog run` to `cog exec` (#2916)
* rename `cog run` to `cog exec`
Reclaim the `run` verb for predictions by moving the arbitrary-command
runner to `cog exec`. `cog run` remains as a hidden alias with a
deprecation warning so existing scripts continue to work.
- Rename pkg/cli/run.go -> pkg/cli/exec.go
- Use Cobra Aliases for the backward-compatible `run` alias
- Print deprecation warning to stderr when invoked as `cog run`
- Update integration tests, docs, and architecture references
- Regenerate docs/cli.md and docs/llms.txt
* add deprecation alias test and fix stale AGENTS.md reference
- Add exec_run_deprecated.txtar to verify 'cog run' alias works and
emits a deprecation warning (matches train_deprecated.txtar pattern)
- Fix AGENTS.md: 'build, run, predict' -> 'build, exec, predict'
* use console.Warn for deprecation warning
Switch from raw fmt.Fprintln(os.Stderr) to console.Warn() for the
'cog run' deprecation message, consistent with how warnings are
emitted elsewhere in the CLI.
