fix(shared/apm.md): drop || 'all' fallback so gh-aw substitutes target (#1185)

The bare-expression regex in gh-aw's importInputsExprRegex
(pkg/parser/import_field_extractor.go) only matches
`${{ github.aw.import-inputs.X }}` -- the `|| 'all'` belt-and-suspenders
introduced in #1184 made the regex skip substitution entirely, so
consumer-supplied `with: target: copilot` was silently ignored and the
unresolved expression survived into the lock workflow (yielding empty
runtime env, which apm then treats as the default).

The schema default `default: all` declared in import-schema is already
applied by gh-aw's applyImportSchemaDefaultsFromFrontmatter BEFORE
substitution, so the fallback was redundant anyway. Dropping it lets
gh-aw substitute literal values into the lock workflow.

Verified: `gh aw compile` now emits `target: all` (literal) into the
two consuming lock workflows. With `with: target: copilot` set on the
import, gh-aw will substitute `target: copilot` instead.

Closes #1185.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: Daniel Meppiel