Archive the Python 3.15 brief

The work it describes has shipped: mcpyrate 4.3.0, unpythonic 2.4.0 and pyan
2.7.0 are all released with 3.15 support.

Cross-references between the three briefs are updated to the new paths, since
all three moved together and would otherwise point at nothing.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

Author: Technologicat

briefs/python-3.15-support.md briefs/done/python-3.15-support.mdbriefs/python-3.15-support.md renamed to briefs/done/python-3.15-support.md

@@ -1,8 +1,8 @@
 # CC Brief: Python 3.15 support (unpythonic side)
 
-Companion to `mcpyrate/briefs/python-3.15-support.md`, which carries the full AST survey and the expander-side work. This one is the unpythonic half, written to stand on its own for anyone looking only at this repo.
+Companion to `mcpyrate/briefs/done/python-3.15-support.md`, which carries the full AST survey and the expander-side work. This one is the unpythonic half, written to stand on its own for anyone looking only at this repo.
 
-Three fleet projects read the Python AST directly and so are the ones a CPython minor version can break: `mcpyrate`, `unpythonic`, and `pyan` (which has its own brief at `pyan/briefs/python-3.15-support.md`, and is the only one with a confirmed 3.15 crash).
+Three fleet projects read the Python AST directly and so are the ones a CPython minor version can break: `mcpyrate`, `unpythonic`, and `pyan` (which has its own brief at `pyan/briefs/done/python-3.15-support.md`, and is the only one with a confirmed 3.15 crash).
 
 ## Context