chore: add Codex project context and agent guidance

.agents/skills/kst4contest-change/references/domain-and-callsigns.md

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+# Domain, callsigns and bands
+
+## Chat-member identity
+
+Full callsign variants can be distinct chat identities.
+
+Examples:
+
+```text
+DN9APW
+DN9APW-2
+DN9APW-70
+```
+
+Do not globally strip suffixes when identifying chat members.
+
+Category is also part of identity. A practical key is conceptually equivalent to:
+
+```text
+FULL_CALLSIGN|CATEGORY
+```
+
+Do not allow messages from one category to attach to a same-looking member in another category.
+
+## Base-call operations
+
+Some features intentionally operate on the base callsign.
+
+Confirmed examples:
+
+### Worked state
+
+Worked status is shared across suffix variants of the same base call.
+
+If the base station has been worked on the relevant basis, variants such as `CALL-2`, `CALL-70`, `CALL-144`, `CALL-432` should not become independent worked identities merely because of the suffix.
+
+### Monitoring
+
+Monitoring a station entered as `DN9APW-2` or `DN9APW-70` should monitor the base call `DN9APW`.
+
+This is intentional: a user monitoring another station's skeds should not have to create one monitor entry per SSID.
+
+Do not extend base-call matching to unrelated features without approval.
+
+## Suffix semantics
+
+Do not assume a suffix always means a band or category.
+
+Historical examples have shown the same base calls with different suffix conventions in different chat categories.
+
+Therefore:
+
+- preserve exact full-call identity where needed;
+- normalize only for explicitly approved base-call features;
+- never infer missing band/category semantics from the suffix alone.
+
+## Categories
+
+KST4Contest's central VHF/UHF usage focuses on ON4KST categories 2 and 3.
+
+However, other categories can occur.
+
+Rules:
+
+- unsupported/uninteresting categories must be ignored or handled safely;
+- they must not produce index, switch or null errors;
+- do not let their values contaminate category-2/category-3 band logic.
+
+## Band availability
+
+Band activity can be derived from several signals including name/text parsing and explicit/manual information.
+
+Confirmed invariant:
+
+`NOT-QRV` overrides positive availability indications.
+
+Known-active-band and `B+` handling should use one consistent interpretation across the program.
+
+Do not implement separate slightly different parsers in multiple UI/features if a shared existing mechanism is available.
+
+## Current QRG
+
+Features that depend on propagation/band/frequency should use the current relevant QRG or the approved band calculation.
+
+Never silently reintroduce a universal hardcoded 144 MHz fallback.
+
+When a frequency is ambiguous and no approved fallback exists, ask rather than guessing.