feat(config): lens sections - a repository's own text appended to a shipped charter

A lens has one charter, the one this pipeline ships; what a repository
knows that is not general to the lens - its platform and layers, the
record it keeps its decisions in, what its users already have, the
calibration entries that are its own - goes in a file the config names
for the lens (lenses.<name>.section), and Charter appends it after one
blank line. The charter sha in every reuse key covers the combined text.

The config refuses a path outside the repository - absolute, or
climbing out with `..` - by the check slots.dir had, now one both
settings share; a file missing or one the run could not use (a dangling
link, unreadable, not UTF-8) is a problem too, checked through the
reader Charter uses (Prompt::ReadSection), so a section is never
dropped in silence, and a name that is no lens is the unknown key it
always was. Nothing lists a directory, so what lies beside the files is
no concern of the config's: a directory listed would have to tolerate or
refuse every foreign entry. The run logs which charters got one.

The better-solution charter gains a consumer's general entry
(2026-10-31: a mechanism reworked a second time in one submission is the
signal to derive the fix at every layer the data crosses). The procedure
names the section wherever it names the charter: among the config's keys,
in the lens inputs, in the reuse rule (an edit to either re-runs the lens)
and in the feedback loop, which routes a repository's own lesson to its
section and a general one to the shipped charter.

tests/Goldens/derive.py

@@ -166,5 +166,6 @@ g['gates'] = new_gates
 
 # ---- the charter ----------------------------------------------------------------------------
 g['prompts']['charterSha']['repo-facts.md'] = sha((W / 'process/lenses/repo-facts.md').read_text())
+g['prompts']['charterSha']['repo-better-solution.md'] = sha((W / 'process/lenses/repo-better-solution.md').read_text())
 G.write_text(json.dumps(g, indent=1, ensure_ascii=False) + '\n', encoding='utf-8')
 print('written', G)