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.

interfaces/Crafter.Review-Config.cppm

@@ -59,6 +59,13 @@ export namespace Crafter::Review::Config {
     std::vector<std::string> SlotCommands(const Json& config, std::string_view which, std::span<const std::int64_t> slots);
     // The first login fixture that exists, relative to the repository root.
     std::optional<std::string> LoginFixture(const Json& config, const std::filesystem::path& repo);
+    // The repository's own lens sections (`lenses.<name>.section`, resolved
+    // against the repository root), keyed by lens file: each a file the
+    // config names inside the repository (a relative path, no `..` part).
+    std::map<std::string, std::filesystem::path> LensSectionsOf(const Json& config, const std::filesystem::path& repo);
+    // What is wrong with those files: one missing, or one
+    // Prompt::ReadSection cannot use.
+    std::vector<std::string> SectionProblems(const Json& config, const std::filesystem::path& repo);
     // The example a consumer starts from (process/templates/config.json is
     // this, rendered).
     Json Example();