fix(projects): name the corrected spec.workflow.file in both missing-workflow errors

[nilsonfh]

Summary

bd 6ns.6, a follow-up from 6ns.2 part 2b. A spec.workflow.file that names no file is reported in two places. They now report the same located, remediating message, and it names the corrected declaration when one can be derived.

Before

• load_project gave the project file, the key, the path it tried and the expected form, but never the fix.
• gridalyn project validate gave only workflow file does not exist: <path>.

After (both)

<project.yaml>: spec.workflow.file names 'projects/my_case/workflow.yaml', which does not exist at <path> -- expected a path to the Workflow YAML, relative to the project directory whatever spec.pathBase says, e.g. file: workflow.yaml; declare 'workflow.yaml'

Changes

• loader.build_missing_workflow_message is shared by both call sites. The correction is appended only when the declaration repeats projects/<study>/, the pre-6ns.2 form of a pathBase: repo study, and the corrected file exists. The builder lives in the loader because it uses the loader's field hints; path_contract cannot import the loader without a cycle.
• path_contract.build_corrected_declaration was the private _suggest, now public so the loader reuses the path gate's derivation instead of copying it. build_* is the documented verb for a deterministic in-memory construction, agreed with the module's author.
• validation._read_workflow_data takes the project.yaml path and checks is_file(), as the loader does.
• docs/reference/workflow-yaml.md: the 2026-09-11 note now says the error names the corrected declaration and that validate reports the same message. One ledger line was re-derived through the tool.

Verification

• Tests: four tests in tests/test_project_error_messages.py. Against unchanged main, the two new behaviours fail as they should: the named correction, and validate's message equalling the loader's. The two guards against over-suggesting pass.
• Targeted tests: 98 passed across 8 files: error messages, path contract, workflow, sense checks, import hygiene, layer direction, doc instructions, verb-prefix conventions.
• mypy: the three ratchets, run as the pre-push hook runs them, stay at 116/116, 846/846, 12/12.
• Lint and ledger: pinned black/isort/flake8 clean; doc instruction ledger OK.
• Full suite: runs in this PR's CI. It is not run locally while a long flagship run occupies this machine.
• Review: requested from the path-contract module's author, who agreed the public name.

🤖 Generated with Claude Code