Support non-Markdown text files in `zest-dev dump`

[nettee]

Problem

zest-dev dump archives Markdown files from a spec directory, but non-Markdown text artifacts such as SQL scripts are not preserved. A downstream archive workflow may then see the archive issue as complete and delete the source directory, silently losing repository-only text files.

Desired behavior

• Recursively archive UTF-8 text files in addition to Markdown, including at least .sql, .json, .yaml, .yml, .txt, and shell scripts.
• Preserve each file's relative path and complete contents.
• Use a representation that safely handles content containing Markdown fences.
• Treat unsupported or binary files as an observable failure; do not report a successful dump while silently omitting them.
• Do not signal dump completion until every file in the spec directory has either been preserved or caused an explicit failure.
• Keep existing Markdown dump behavior compatible.

Acceptance criteria

1. A fixture containing spec.md and a nested validation.sql is dumped with both relative paths and byte-equivalent text contents recoverable from the archive issue.
2. A spec containing an unsupported binary file exits non-zero and identifies the unsupported path.
3. A partial upload or failed comment creation exits non-zero rather than reporting success.
4. Tests cover mixed Markdown/text directories and text containing triple-backtick sequences.
5. The supported text-file and unsupported-file behavior is documented.