feat(fleet): add session-detritus archiver

[hyperpolymath]

Summary

• add the review-tier-ready recipe-archive-session-detritus fix script
• constrain candidates to Git-tracked files below LOCAL standards REGISTRY homes
• archive matched session reports under docs/archive/session-detritus while preserving their original relative paths
• exempt external homes, existing archives, examples, templates, and test fixtures
• refuse destination collisions and mismatched recipe findings
• register both the SessionDetritus category and recipe with the fleet dispatcher

Verification

• bash syntax, ShellCheck, registry JSON, and git diff checks pass
• hermetic smoke test proves dry-run behavior, exactly two planted positive matches, negative boundaries, staged Git renames, second-run idempotence, and collision refusal
• read-only dry run against the live standards checkout reports zero current tracked matches in LOCAL spec homes

The live zero is scoped to tracked files inside LOCAL registry homes in the current standards checkout; the planted positive control demonstrates that the detector can produce an unsafe result.

Refs hyperpolymath/standards#498
Refs hyperpolymath/standards#479

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