fix(dlt): surface underlying error and hint when attaching to pipeline fails

[nkwork9999]

Description

Fixes #5660.

When sqlmesh init -t dlt --dlt-path <path> cannot attach to the pipeline, the underlying dlt error is swallowed and replaced with an uninformative Could not attach to pipeline <name> message.

The root cause of the confusion in #5660 is that --dlt-path must point to dlt's pipelines working directory (where pipeline state is stored, by default ~/.dlt/pipelines), not the directory containing the pipeline scripts — and both the help text and the swallowed error gave the user no way to discover that.

This PR:

• Surfaces the original dlt exception and the directory that was searched.
• Adds a hint to omit --dlt-path when a pipeline with the given name exists in the default pipelines working directory (the exact scenario in [bug] DLT Error: Could not attach to pipeline #5660).
• Clarifies the --dlt-path help text, docstring, and CLI docs.

Before:

Error: Could not attach to pipeline sushi

After:

Error: Could not attach to pipeline sushi.
Searched in: ./scripts
Pipeline with `pipeline_name=sushi` and `pipelines_dir=./scripts/sushi` could not be restored: No local pipeline state found in './scripts/sushi'.
...
Hint: A pipeline named 'sushi' exists in the default pipelines working directory '/Users/nk/.dlt/pipelines'. Note that --dlt-path must point to the directory where DLT stores pipeline working state (by default ~/.dlt/pipelines), not the directory containing your pipeline scripts. Try omitting --dlt-path.

Test Plan

• uv run --extra dev --extra web --extra slack --extra dlt --extra lsp make style
• uv run --extra dev --extra web --extra slack --extra dlt --extra lsp pytest tests/cli/test_cli.py -q (the one failure, test_plan_very_verbose, also fails on main in my environment and is unrelated)
• Extended the existing test_dlt_pipeline test to assert that the error reports the searched directory and the --dlt-path hint.

Checklist

• I have run make style and fixed any issues