feat(github): collect GitHub issue fields and map them onto domain issues (#9038)

* feat(github): collect GitHub issue fields and map them onto domain issues

GitHub issue fields are organization-level structured issue metadata that
went generally available on 2026-07-02. They are typed, mutually exclusive
within a field, and shared across every repository in the organization —
which is what teams currently approximate with `type:`-style labels.

Collects issue field values and lets a scope config map a field onto an
issue column, where it takes precedence over the existing label regexes.

- New table `_tool_github_issue_field_values`, one row per issue per field,
  carrying a queryable text form of the value alongside the original JSON.
- New subtasks Collect/Extract Issue Field Values, both disabled by default
  so existing pipelines are unaffected until a mapping is configured.
- New scope config keys issueFieldPriority, issueFieldSeverity,
  issueFieldComponent, issueFieldStoryPoint and issueFieldDueDate, each
  holding a field *name*.

The mapping is applied in the issue convertor rather than written back into
`_tool_github_issues`: the collector iterates that table to build its request
URLs, so a subtask that both read and wrote it was a cycle in the subtask
graph. Converting instead also keeps the tool layer as raw GitHub truth and
avoids adding columns there.

A 404 from the field-values endpoint is treated as "no field values" so an
organization that has never configured issue fields, or a token that cannot
see them, does not fail the whole task.

* test(github): add an e2e dataflow test for issue field values

Covers extraction and the scope config mapping end to end against a real
database, which the unit tests could not reach.

Extraction asserts the value normalisation per data type: a single_select
resolving to its option name and colour, an integral number rendering as
"5" rather than "5.0", a fractional number keeping its precision, a
multi_select joining option names while keeping the raw JSON array, and a
null value producing an empty value.

Conversion asserts the mapping reaches the domain issue -- priority,
component, story point and due date -- and, for the case that matters,
that an unparseable value is skipped with a warning rather than failing
the task or writing a wrong value: issue #7 carries "soon" in a date
field and a null priority, and comes out with neither set while the other
issues are untouched.

* fix(github): register the issue field values table in GetTablesInfo

Test_GetPluginTablesInfo compares the plugin's declared tables against the
ones its migrations create, and the new table was missing from the list:

  table_info_test.go:121: The following tables are not returned by the
  TablesInfo method
      _tool_github_issue_field_values

Adds GithubIssueFieldValue to Github.GetTablesInfo(). Verified inside the
mericodev/lake-builder image the unit-test job uses, since the plugins
package needs libgit2 to build.