Orbit/Galaxy MCP: no way to delete/purge datasets from a history

A tester asked Orbit to permanently delete/purge failed outputs from a history (a common cleanup chore). Orbit can't -- there's no MCP delete/purge endpoint -- so it offered to run `__FILTER_FAILED_DATASETS__` on the collections and otherwise gave manual Web-UI instructions for purging the individual failed datasets.

(As with the other reports in this set, the model attributed the limitation partly to "sandbox security policy," which is a misattribution -- the real gap is a missing MCP tool.)

This is a galaxy-mcp tool-surface gap; tracking here since that's where testers feel it. One of a small set of "missing Galaxy operation" reports filed together -- see siblings for Data Libraries, workflow import, and paired-collection build.

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Filed together as the missing-Galaxy-operation set: #226 (Data Libraries), #227 (workflow import), #228 (dataset purge), #229 (paired-collection build).