Added CODE_CONVENTIONS.md and linters to enforce it, and applied it across the codebase

CODE_CONVENTIONS.md sets out the conventions for Go code in this
repository: naming, layout, comments, errors, logging, configuration and
tests. .golangci.json enables the linters that check the parts that can be
checked (each limited to the rules the conventions use); the rest is
applied by hand.

Applied across the codebase:
- Naming: full-word names in application code, initialisms in one case
  (BuyerID, PrintJSON, PrintYAML, ocdsGetBuyerID), no names shadowing
  built-ins (min/max are minGap/maxGap), mustRaw renamed rawJSON as it
  returns nil rather than panicking, and config.ConfigLevelToLogLevel
  renamed LevelToLogLevel.
- Layout: lines kept to 80 characters where possible, assignments taken
  out of if and for statements that don't fit, wrapped arguments all on
  one line or one per line, steps separated by blank lines, no else after
  a return, function and struct literals wrapped.
- Comments: doc comments on every exported name (including each section's
  Map, Flatten, TypeOf, ValueOf and SetValue), section banners, British
  spelling.
- Errors: a created error starts with its package name, a wrapping error
  adds only what was being done, sentinels in one commented var block,
  errors.Is and errors.As in place of == and type assertions, and no
  err := shadowing of a named result. That fixes the mock server losing
  its access log's close error.
- Logging: the fetcher passes its workers an untagged logger, so
  findatender's lines are tagged with their own component rather than
  with fetcher's as well; cmd/mock-fts logs "stopped" with slog.
- Configuration: util.WithTimeout (moved from the websocket package)
  applies a timeout of 0 or less as no limit, used for the websocket
  write and ping, TCP close and tcpserver shutdown timeouts; every setting
  documents what 0 or empty means, including the rate limit rule's keys
  in internal/config/README.md.
- Tests: related tests merged into table-driven ones with named cases.

Separately from CODE_CONVENTIONS.md, internal/store/README.md gains a soft
SQL style convention for the queries in internal/store/queries.go, and the
queries are reformatted to follow it.

internal/store/README.md

@@ -78,6 +78,18 @@ Reads select whole rows (`SELECT *`, `t.*`, `c.*`) which map to a Go struct. sql
 
 `ListOpportunities` is assembled by `listOpportunitiesQuery` from fragments in `queries.go`: a base `SELECT`, one `filterOpportunity*` clause per filter that is set, and the ordering and paging tail. Each filter fragment takes exactly one argument, except the `filterOpportunityMark*` fragments, which take none (`MarkUnset` puts `NOT` in front of them). Adding a filter means one fragment and one line in `listOpportunitiesQuery`. The classifications and the marks for a page of opportunities are then loaded with one `IN (...)` query each.
 
+### SQL style
+
+A soft convention for the SQL in `queries.go`, followed so the queries look alike and read easily. A query that reads better another way can depart from it.
+
+- A statement starts on the line after the opening backtick. A fragment combined into another query (`filterOpportunity*`) starts straight after it.
+- Keywords are uppercase (`SELECT`, `INSERT INTO`, `ON CONFLICT`); table and column names are the lowercase snake_case of the schema.
+- A short statement may stay on one line (`SELECT t.* FROM opportunities t WHERE t.uuid = ?`). A longer one has one clause per line, each at the start of the line: `SELECT`, `FROM`, `JOIN`, `WHERE`, `ORDER BY`, `LIMIT`.
+- A column list that doesn't fit on one line goes inside its brackets, indented one tab and filled to about 80 characters per line. The `VALUES` list wraps at the same columns, so each line of values matches the line of columns above it.
+- `ON CONFLICT … DO UPDATE SET` puts one `column = excluded.column` per line, indented one tab.
+- A subquery (`EXISTS (…)`, `IN (…)`) is indented one tab inside its brackets, with the closing bracket at the start of a line.
+- Tables are aliased consistently: `t` for `opportunities`, `c` for `classification_codes`, `tc` for `opportunity_classifications`, `m` for `opportunity_marks`.
+
 ## Store API
 
 ```go