fix(mcp): name the artifact phase when a post-publish export fails

[DeusData]

Distilled from #1789 by @umi008 (Ulises Millan Guerrero), carried here with Co-authored-by. #1789 stays open until this merges.

The fix

When artifact export failed after the graph database was already published, the MCP error path returned the generic hint regardless:

Pipeline failed. Check repo_path exists and contains source files.

So a caller whose export failed — a read-only checkout, typically — was told to check a repository path that was perfectly fine, while the one log line naming the real phase sat somewhere they were not looking. The pipeline already recorded its own export error; it simply never reached the caller.

The failing phase is now carried out of the pipeline into the MCP error, so the message names artifact export when that is what failed. The existing fail-hard contract is unchanged: this changes what the caller is told, not whether the run fails.

Why a distill rather than the original

Five of the nine files in #1789 were openspec/ spec-framework scaffolding. openspec/ does not exist on main, no other contributor uses it, and adopting a spec framework is a project-level decision rather than something that rides along with a bug fix. That was explained on 2026-09-02, and the code half was approved on merit the same day — the scaffolding is the only reason it did not land then.

This PR is that code half, unchanged, rebased onto current main: 4 files, +110 lines, no openspec/.

Verification

• build/c/test-runner pipeline mcp → 597 passed, 1 failed.
• The single failure is tool_check_index_coverage_freshness_uses_indexer_mtime_source_issue1714, which fails identically on unmodified main (confirmed by reverting these four files in the same worktree and re-running: 317 passed, 1 failed, same test). It is unrelated to this change and is being tracked separately.
• Co-authored-by verified with git log --format='%(trailers:key=Co-authored-by)' rather than grep, since a blank line hides a trailer from git's parser while grep still finds it.

Credit for the diagnosis, the fix and the test coverage is entirely @umi008's.