Rename `CLI_DIRECT_READ` to `DIRECTED_READ` and make read/write

[apstndb]

Rename CLI_DIRECT_READ to DIRECTED_READ and make read/write

Context

Part of #47 (java-spanner compatibility). Currently CLI_DIRECTED_READ is a read-only variable with a name that was previously a typo (CLI_DIRECT_READ). It needs to match the upstream DIRECTED_READ name and support read/write access.

Upstream References

• java-spanner (ClientSideStatements.json): DIRECTED_READ with SET/SHOW support (read/write)
• go-sql-spanner: Not present as a runtime variable (ExecOptions only)

Changes Required

Variable Rename

• Rename CLI_DIRECTED_READ → DIRECTED_READ
• Drop CLI_ prefix to match java-spanner naming

Make Read/Write

• Currently read-only (set only via --directed-read CLI flag)
• Add SET support: reuse parseDirectedReadOption() from session.go as setter parser
• Keep SHOW support

CLI Flag

• Keep --directed-read CLI flag working (sets initial value)

Backward Compatibility

• No backward compatibility required per project philosophy

Acceptance Criteria

• SET DIRECTED_READ = '...' works with directed read option string
• SHOW VARIABLE DIRECTED_READ works
• --directed-read CLI flag still works
• CLI_DIRECTED_READ is fully removed
• make check passes